Add American V8 preset, carb fuel transport, and adaptive desktop audio

CARB_PROGRESS.md

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+# Carburetor work in progress
+
+Resumed on 2026-10-23 to fix throttle-release crashes and remove scheduled pops.
+All changes are uncommitted. The historical results below predate that revision.
+
+Current revision: bounded slide aperture to avoid negative roundoff near its
+seat; removed the hardcoded firing intervals and forced late spark. Exhaust
+combustion was subsequently removed at the user's request due to lag and limited
+audible benefit. Fuel remains transported through pipes without reacting.
+Cylinder combustion consumes air in proportion to burned fuel.
+Warm-running startup now seeds fuel in the intake, crankcase and cylinder.
+Pilot effective diameter is .40 mm with .08 base/.3 maximum idle slide opening.
+The revised transient passes: peak 12954 RPM, idle 2583 RPM, peak audio .0985.
+
+## Implemented
+
+- Added `Carburetor`, with effective pilot, needle and main jet restrictions.
+- Jet flow uses simulated air mass flow, air density, venturi dynamic pressure,
+  static pressure drop, fuel density, discharge coefficients and float-bowl
+  supply. It is not a cylinder-side target-AFR calculation.
+- Added wall-film deposition and evaporation during throttle transients.
+- Added fuel as a conserved transported species to pipe cells, boundaries,
+  0-D volumes, crankcase, cylinder and turbo internal transfers.
+- Carb fuel enters the intake plenum, then moves through the reed, crankcase,
+  transfer pipe and cylinder with the simulated gas flow.
+- Cylinder heat release is limited by transported fuel, trapped oxygen and a
+  basic 7:1--24:1 flammability window. Rich excess remains unburned.
+- Disabled electronic overrun fuel cut for the two-stroke. A deterministic
+  sparse late-fire cadence overlaps exhaust-port opening to make separated
+  coast-down bangs through the pressure/audio solver; no sound sample is mixed.
+- Added fuel conservation and downstream transport tests, carb bounds/cadence
+  checks, transient firing-count checks, benchmark AFR columns and desktop carb
+  labeling.
+
+## Last verified state
+
+- Before the final throttle-slide geometry change, the entire 10-test CTest
+  suite passed.
+- That transient reached about 15,954 RPM, recovered to about 3,151 RPM, and
+  produced 11 late firings in the first second after release with no clipping
+  or pipe positivity repairs.
+- After replacing the old butterfly-area formula with a circular-segment slide
+  aperture and reducing the idle slide calibration, the project and desktop
+  rebuilt successfully and `fluidsim_two_stroke_physics` passed.
+
+## Current task / next step
+
+The `fluidsim_two_stroke_regression` run after the slide-area change was stopped
+mid-run when work was paused. Run it first, then run full CTest. If idle recovery,
+mixture, or overrun firing count moved, tune `idle_base_opening`,
+`idle_max_opening`, and the effective jet dimensions in `Carburetor.hpp`.
+Re-run `fluidsim_two_stroke_benchmark` afterward and update the YZ125 verification
+numbers if the slide geometry changed them.
+
+The most relevant files are:
+
+- `include/FluidSim/Carburetor.hpp`
+- `src/carburetor.cpp`
+- `src/two_stroke_scenario.cpp`
+- `src/two_stroke_cylinder.cpp`
+- `include/FluidSim/Fluid.hpp`
+- `tests/two_stroke_physics.cpp`
+- `tests/two_stroke_regression.cpp`