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/fivehundred.toml

@@ -0,0 +1,68 @@
+# Engine definition. Dimensional keys state their units.
+schema_version = 1
+id = "fivehundred"
+name = "500 cc / big single"
+category = "Two-stroke / 500 cc"
+description = "Concept single / transported carb fuel"
+order = 16
+cycle = "two_stroke"
+
+[two_stroke]
+initial_rpm = 3000.0
+starter_torque_nm = 8.0
+maximum_load_torque_nm = 10.0
+crankshaft_inertia_kg_m2 = 0.04002316
+bore_mm = 86.0
+carburetor_diameter_mm = 41.0
+crankcase_clearance_cc = 600.3475
+exhaust = "racing"
+exhaust_cell_length_mm = 15.0
+exhaust_duration_deg = 190.0
+exhaust_width_mm = 57.15746
+friction_bar = 0.4
+friction_bar_per_mps = 0.09
+friction_bar_per_mps2 = 0.0009
+idle_minimum_advance_deg = -10.0
+idle_slide_opening = 0.045
+idle_spark_gain = 0.05
+jet_scale = 1.0
+main_jet_scale = 1.7504
+minimum_burn_s = 0.001270166
+needle_jet_scale = 1.06
+needle_low_scale = 3.04
+needle_mid_scale = 0.8
+needle_off_idle_scale = 12.0
+needle_quarter_scale = 1.4
+needle_tip_scale = 5.0
+needle_transition_scale = 9.0
+needle_upper_scale = 0.76
+pilot_jet_scale = 0.75
+pilot_tip_scale = 1.0
+pilot_transition_scale = 1.0
+rod_mm = 174.6478
+stroke_mm = 86.0
+transfer_duration_deg = 130.0
+transfer_width_mm = 79.38537
+trapped_compression = 8.2
+chamber_sections_mm = [[254.0332, 57.15746, 57.15746], [285.7873, 57.15746, 95.26245], [301.6644, 95.26245, 166.7093], [206.402, 166.7093, 166.7093], [333.4185, 166.7093, 34.92956], [381.0498, 34.92956, 34.92956]]
+
+[ecu]
+idle_control = "fixed"
+idle_rpm = 3000.0
+idle_max_opening = 0.3
+throttle_time_constant_s = 0.0002
+throttle_rate = 5000.0
+rev_limit_enabled = false
+rev_limit_fuel_cut_enabled = false
+overrun_cut_rpm = 5000.0
+overrun_resume_rpm = 3500.0
+overrun_fuel_cut_enabled = false
+overrun_ignition_cut_enabled = true
+ignition_map = [[0, 10], [3000, 18], [6000, 24], [9000, 18], [11000, 12], [13000, 6], [15000, 2], [18000, 0]]
+
+[vehicle]
+primary_reduction = 3.368421
+final_drive_ratio = 3.769231
+mass_kg = 170
+wheel_radius_mm = 331.3
+gear_ratios = [2.384615, 1.933333, 1.588235, 1.352941, 1.176471, 1.055556]