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.

engines/single-450.toml

@@ -0,0 +1,136 @@
+# Engine definition. Dimensional keys state their units.
+schema_version = 1
+id = "single-450"
+name = "450 cc / carbureted four-stroke thumper"
+category = "Singles / four-stroke"
+description = "40 mm carb / mild valve timing / fixed idle stop"
+order = 17
+cycle = "four_stroke"
+
+[engine]
+bank_angle_deg = 60.0
+collector_volume_litres = 0.25
+cylinders = 1
+initial_rpm = 1960.0
+initialize_phased_charge = true
+layout = "inline"
+maximum_load_torque_nm = 30.0
+minimum_substeps = 1
+phase_degrees = [0.0]
+pipe_damping = 1.0
+plenum_volume_litres = 1.0
+starter_torque_nm = 11.21926
+throttle_diameter_mm = 40.0
+topology = "collector"
+
+[cylinder]
+afr = 14.7
+ambient_temperature_k = 300.0
+bore_mm = 96.0
+burn_duration_deg = 60.0
+burn_start_deg = 5.0
+compression = 10.0
+energy_variation = 0.05
+exhaust_close_deg = 370.0
+exhaust_open_deg = 145.0
+exhaust_valve_count = 1
+exhaust_valve_diameter_mm = 30.72
+exhaust_valve_lift_mm = 8.640001
+fuel_lhv_j_per_kg = 4.4e+07
+heat_transfer = 100.0
+intake_close_deg = 565.0
+intake_open_deg = 355.0
+intake_valve_count = 1
+intake_valve_diameter_mm = 36.48
+intake_valve_lift_mm = 9.6
+misfire_probability = 0.0
+rod_mm = 111.6
+stroke_mm = 62.0
+wall_area = 0.02
+wiebe_a = 5.0
+wiebe_m = 2.0
+
+[crankshaft]
+friction_constant_nm = 1.795081
+friction_quadratic_nm_s2 = 0.0
+friction_viscous_nm_s = 0.001795081
+inertia_kg_m2 = 0.05385243
+
+[ecu]
+idle_base_opening = 0.045
+idle_control = "fixed"
+idle_ki = 4e-05
+idle_kp = 0.0001
+idle_max_opening = 0.25
+idle_minimum_advance_deg = -5.0
+idle_rpm = 1400.0
+idle_spark_gain = 0.02
+ignition_map = [[0.0, 10.0], [15000.0, 40.0]]
+overrun_cut_rpm = 3500.0
+overrun_fuel_cut_enabled = false
+overrun_ignition_cut_enabled = true
+overrun_resume_rpm = 2400.0
+power_afr = 12.8
+rev_limit_enabled = true
+rev_limit_fuel_cut_enabled = false
+rev_limit_rpm = 7000.0
+rev_resume_rpm = 6750.0
+stoichiometric_afr = 14.7
+throttle_closing_rate = 0.0
+throttle_closing_time_constant_s = 0.0
+throttle_rate = 1000.0
+throttle_time_constant_s = 0.001
+
+[vehicle]
+air_density = 1.225
+clutch_capacity = 98.72945
+clutch_engagement_rate = 8.0
+drag_coefficient = 0.7
+final_drive_ratio = 2.7
+frontal_area = 0.65
+gear_ratios = [4.1, 2.7, 2.0, 1.55, 1.25, 0.98]
+gravity = 9.80665
+mass_kg = 240.0
+primary_reduction = 1.8
+rolling_resistance = 0.015
+rotational_inertia_kg_m2 = 1.2
+wheel_radius_mm = 320.0
+
+[fuel]
+system = "carburetor"
+pilot_jet_diameter_mm = 0.09349564
+main_jet_diameter_mm = 2.091829
+needle_jet_diameter_mm = 2.15284
+needle_low_scale = 0.944
+needle_mid_scale = 0.564
+needle_off_idle_scale = 0.995
+needle_quarter_scale = 0.835
+needle_tip_scale = 0.97
+needle_transition_scale = 1.6
+needle_upper_scale = 0.65
+bowl_capacity_kg = 0.1
+bowl_refill_kg_per_s = 0.1
+
+[[pipes]]
+id = "intake"
+cells = 4
+diameter_mm = 75.0
+length_mm = 200.0
+
+[[pipes]]
+id = "runners"
+cells = 5
+diameter_mm = 35.0
+length_mm = 250.0
+
+[[pipes]]
+id = "primaries"
+cells = 13
+diameter_mm = 38.0
+length_mm = 650.0
+
+[[pipes]]
+id = "tailpipe"
+cells = 30
+diameter_mm = 60.0
+length_mm = 1500.0