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.

include/FluidSim/Carburetor.hpp

@@ -5,21 +5,30 @@
 
 namespace fluidsim::port {
 
-// Lumped slide-carburetor model for the crankcase-scavenged two-stroke. The
+// Lumped carburetor shared by two-stroke and four-stroke scenarios. The
 // jets use liquid-orifice flow from a float bowl into the measured venturi
 // depression. Delivered fuel is injected into the downstream fluid volume and
 // is then transported as a conserved species; this class never doses a cylinder.
-class Carburetor {
-public:
+struct CarburetorConfig {
     float fuel_density{740.0f};
     float pilot_jet_diameter{0.00040f}, main_jet_diameter{0.00150f};
     float needle_jet_diameter{0.00190f};
+    // Needle annulus area corrections at configured slide positions; jets meter
+    // liquid fuel from measured pressure, without AFR feedback.
+    float needle_low_scale{1.f}, needle_mid_scale{1.f};
+    float needle_off_idle_scale{1.f}, needle_transition_scale{1.f}, needle_tip_scale{1.f};
+    float needle_quarter_scale{1.f}, needle_upper_scale{1.f};
+    float pilot_transition_scale{1.f}, pilot_tip_scale{1.f};
     float pilot_discharge_coefficient{0.62f}, main_discharge_coefficient{0.72f};
     float bowl_head_pressure{};
     float film_fraction{0.28f}, film_time_constant{0.11f};
     float bowl_capacity{0.008f}, bowl_refill_rate{0.0025f}; // kg, kg/s
     float overrun_rpm{4200.0f}, overrun_exit_rpm{3300.0f};
 
+};
+
+class Carburetor : public CarburetorConfig {
+public:
     explicit Carburetor(float throat_diameter = 0.0381f);
 
     // air_mass_flow is positive from atmosphere toward the engine.