Load engines from TOML and improve scenario audio and carb tuning

Migrate 32 engine definitions into validated startup-loaded TOML files and separate configuration and snapshots from scenario assembly. Add camera-based stereo propagation from shared pipe geometry, preserve exhaust character, tune carburetor mixtures and idle, use ignition cuts for carb engines, and support returning to the scenario menu.

Validation: full Windows build and all 14 CTest checks passed.

README.md

@@ -123,18 +123,27 @@ The primary source is a linear compact-monopole approximation,
 `p(r,t) = rho_ambient * (d volume_flow/dt)/(4*pi*r)`. Mouth volume flow
 comes from the boundary face velocity and area. Fluid conservation retains the
 actual mass flux; hot/cold density changes are excluded from the acoustic source.
-Intake and exhaust have separate static
-positions, fractional-sample propagation delays, inverse-distance spreading,
-and left/right ear paths. Playback calibration is 200 Pa per device full scale;
+Intake and exhaust sources sit at the open ends of the same world routes used
+to draw the pipes, including both outlets of dual exhausts. The desktop camera
+is the listener: orbit, pan and zoom change its position and ear orientation.
+Left and right ears are 18 cm apart, with separate fractional-sample travel
+delays at 343 m/s and inverse-distance spreading. Camera updates smooth over
+20 ms while retaining propagation history and reflection filter state.
+Playback calibration is 200 Pa per device full scale;
 this is not calibrated speaker SPL. A second-order 6 kHz monitoring filter
 reduces sharp sample-to-sample transients without saturation or synthetic bangs.
 A roughly 3.5 Hz device DC blocker and final PCM clipping protection remain.
-Use 1x playback for acoustic evaluation.
+Use 1x playback for acoustic evaluation. Toggle **G** off to hear the full
+distance-related loudness change; automatic gain otherwise compensates for it.
 
 The default environment is a nearby wall. Press **R** to cycle dry, field,
 wall, woodland, and tunnel. Ground/wall image paths provide early reflections;
 the live eight-line feedback-delay network supplies a scene-dependent late
-tail. Woodland scatterers and the hard-surface tunnel images are approximate.
+tail. Early reflections follow the listener geometry, with plane reflections
+disabled when the listener crosses to the other side. The diffuse tail receives
+a delayed centre signal rather than summing the ears, avoiding ear-delay comb
+filtering in the reverb input. Woodland scatterers and the hard-surface tunnel
+images are approximate.
 These are listening profiles, not validated acoustics. Mechanical sounds are
 not implemented.
 Open pipe mouths now use a causal unflanged radiation boundary with frequency-
@@ -149,7 +158,9 @@ for capture commands, current renderer details and limitations.
 Launching `fluidsim_desktop` without `--scenario` opens a small scenario menu.
 Choose the twin, YZ125, Saab B205, or American V8 card, or select with Up/Down
 and press Enter.
-Escape closes the menu. Simulation and audio start after selection. Passing
+Escape returns from a running scenario to the menu, stopping its simulation and
+audio so another scenario can be selected. Escape closes the menu at the category
+screen. Simulation and audio start after selection. Passing
 `--scenario twin`, `--scenario two-stroke`, `--scenario b205`, or `--scenario v8`
 bypasses the menu; `--scene` and `--exhaust` options also apply
 when choosing through the menu.
@@ -353,11 +364,12 @@ catalyst, cold-start temperature model or injector electrical model. Idle
 control can still stall under excessive load and does not impose crankshaft
 speed or add artificial sustaining torque. Existing combustion/thermal models
 remain approximations; ECU changes do not establish their physical accuracy.
-The two-stroke uses a fixed mechanical carb slide stop instead of RPM-controlled
-idle throttle or idle ignition trim. Each carb scenario config sets
-`TwoStrokeConfig::idle_slide_opening` (normalized 0..1); the default is 0.045,
-or 4.5% slide travel. Pedal demand above that stop opens the slide further.
-The resulting idle RPM depends on the simulation and load. Its
+The two-stroke uses a fixed mechanical carb slide stop. With the pedal released,
+the ECU trims ignition timing around the configured idle RPM instead of moving
+the carb stop. Each carb scenario config sets
+`TwoStrokeConfig::idle_slide_opening` (normalized 0..1); the catalog 125 cc
+scenarios now use a calibrated 3% stop. Pedal demand above that stop opens the
+slide further. The resulting idle RPM depends on the simulation and load. Its
 reed is a damped lumped cantilever model with finite opening time and seating
 contacts. Cylinder and crankcase moving-volume work use adiabatic energy changes.
 Jet dimensions are effective flow calibrations rather than measured Mikuni
@@ -492,7 +504,8 @@ equal 400 mm x 45 mm secondaries, then a 50 mm exhaust. Pairings are 1+2 / 3+4
 and 5+7 / 6+8 with this simulation's bank numbering and firing order. Two T
 junctions and a 780 mm x 38 mm tube form the H-pipe. Small configurable port-coupled junction volumes are used by the V8. The intake
 plenum also remains a storage volume.
-`V8ExhaustConfig` configures primary length/diameter and enables the H crossover;
+The TOML `primaries` pipe group configures primary length/diameter, while
+`[junctions] h_pipe` enables the H crossover;
 pass it as the fourth `FourStrokeScenario` constructor argument. The renderer
 routes the primary pairs into Y merges, then the final Y collectors and H-pipe,
 using local pipe diameter and pressure colors without collector boxes.
@@ -505,16 +518,25 @@ The previous header optimization measurements applied to the earlier finite-volu
 The startup browser now scrolls with the mouse wheel, Up/Down, Page Up/Down,
 Home and End. It adapts to the window size and shows a scroll indicator. Click
 an example or press Enter to start it. The examples are defined in
-`ScenarioCatalog.hpp`, grouped into categories with selectable variants. These are configuration comparisons of
+`engines/*.toml`, loaded at startup and grouped into categories with selectable variants. These are configuration comparisons of
 the existing engine models, not measurements of additional production engines.
 
 The V8 currently uses port-coupled finite-volume fittings, configured by
-`V8ExhaustConfig::junction_volume` (default 50 cc per fitting). They use the
+`JunctionConfig::junction_volume` (default 50 cc per fitting). They use the
 existing conservative component/port exchange and have no volume boxes in the
 renderer. The earlier zero-storage Rusanov junction remains available for
 comparison through `BoundarySystem::add_junction`, but is not used by the V8.
 The 4-2-1 piping and the H crossover geometry remain the same.
 
+### TOML engine definitions
+
+Every engine is defined in its own file under `engines/`. Copy an existing
+`.toml` file, change its `id`, name and calibration, then restart to load it.
+No rebuild is required when editing definitions in the active directory.
+Unknown keys, invalid dimensions, malformed timing and duplicate IDs produce
+a file-specific startup error. See [the engine configuration guide](docs/ENGINE_CONFIG.md)
+for units, directory discovery, sections and pipe topology ordering.
+
 ### Engine families and inspired builds
 
 The browser opens at engine categories; click a category to see variants, then
@@ -531,7 +553,7 @@ estimated calibrations and simplified common-plenum/exhaust networks: they are
 not factory replicas, measured torque curves, or turbocharged configurations.
 
 The 8.0 L big-block variant transports carburetor fuel through its intake into
-the cylinders. Its fixed idle stop is configured with `EngineConfig::idle_opening`.
+the cylinders. Its fixed idle stop is configured with `[ecu] idle_base_opening` in its TOML file.
 It reuses the lumped carb model with scaled effective jets; multiple carb barrels,
 floats and cylinder-specific fuel distribution are not resolved. Larger engine
 geometry requires its own mixture and idle calibration.
@@ -544,5 +566,56 @@ chamber is a geometry experiment, not a guaranteed RPM or power improvement.
 
 Launch a specific variant directly with `--example big-carb`, `--example flat6`,
 `--example fifty` or another catalog ID. Carb jet scaling is configurable per
-engine (`EngineConfig::carb_jet_scale`, `carb_pilot_scale`, or
-`TwoStrokeConfig::jet_scale`).
+engine: four-strokes use effective jet diameters in `[fuel]`; two-strokes
+use jet multipliers in `[two_stroke]`.
+
+### Pulse-preserving custom exhausts
+
+The custom inline three, F20 four, inline five and inline six examples use
+unequal-length primaries, 50 cc junction collectors and secondary pipes rather
+than a single large mixing volume. Their pipe damping matches the undamped Saab
+path. Inline fours/fives/sixes use physical firing orders so collector grouping
+preserves pulse timing; combustion remains even-fire.
+
+The custom big-block V8 uses the American V8's cross-plane 1-5-4-2-6-3-7-8 firing
+order and 4-2-1 headers with an H crossover, retaining its own displacement and
+carburetor. V10/V12 examples retain separate banks through junction collectors,
+secondary pipes and two radiating outlets, including both outlets in the spatial
+audio renderer. The existing Saab, American V8, single and twin presets retain
+their exhaust paths.
+
+Capture any catalog entry at controlled RPM for comparison:
+
+```powershell
+./out/build/windows-gcc/fluidsim_audio_capture.exe --example v12 --rpm 3000 --warmup 0.6 --seconds 2 --throttle 0.55 --scene dry --out out/audio/v12
+```
+
+See [custom exhaust verification](verification/CUSTOM_EXHAUST_CHARACTER.md) for
+fixed-RPM comparison results.
+
+### Carburetor calibration
+
+All seven two-stroke catalog variants and both carbureted four-strokes have
+separate pilot, main-jet and needle-taper calibrations. Progressive pilot metering
+fills the 5% throttle transition after lowering the 125 idle stops. The smaller four-stroke
+single (`single-450`) now uses a 40 mm carburetor, transported fuel and milder
+355/565-degree intake and 145/370-degree exhaust timing. The 650 cc single keeps
+EFI. The big-block idle opening is 7%; the straight-pipe 125 uses 3%. The racing
+125 and its chamber variants use the calibrated 3% mechanical slide stop.
+
+Needle annulus area is interpolated continuously across slide positions; liquid
+fuel still comes from the simulated jet pressure drop and float bowl. There is
+no AFR feedback or cylinder-side fuel correction. The displayed carb AFR is the
+uncapped ratio of filtered air flow to delivered fuel flow.
+
+`fluidsim_carb_calibration --check` tests every carbureted catalog scenario at
+idle, 5%, 10%, 15%, 25%, 50%, 75% and full pedal on an imposed-speed dyno, then
+releases the dyno for an idle run. It fails on AFR outside the calibration
+bounds, stalling, nonfinite RPM/AFR, pipe positivity repairs, a changed racing
+125 stop, or loss of transported carb fuel in the 450 single. Run it through
+`ctest --preset windows -R carb_calibration --output-on-failure`.
+
+The tool also prints CSV diagnostics: run without arguments for the four main
+operating points, or use `--sweep single-450` for a denser sweep of one scenario.
+See [carb calibration results](verification/CARB_CALIBRATION.md) for operating
+conditions, AFR values and the distinction between carb and recirculated charge.