Sound: two-layer engine architecture + rotary pipeline for the RX-7

- SoundController rebuilt around idle+rev loop crossfading (real racing
  game approach): idle layer barely pitch-shifts (keeps engine character),
  rev layer carries the speed range; crossfade window 5%-45% of top speed
- Per-car CAR_SOUNDS table so the RX-7 gets real 13B rotary loops while
  other cars keep the generic set
- docs/ROTARY_SOUND.md: full record->loop->upload pipeline for capturing
  the owner's REAL RX-7 (extract audio from video, zero-crossing cut +
  equal-power crossfade seam, normalize, ogg, Studio upload, wire IDs)
- Toolbox rotary rips evaluated and rejected: unauditionabe off-platform,
  short/unloopable, murky rights; the real car beats any rip

docs/ROTARY_SOUND.md

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+# Real 13B rotary engine sound (rx7_fc)
+
+The RX-7 must sound like a rotary. The pipeline: the owner's own recordings →
+processed into two seamless loops on this workstation → uploaded in Studio →
+IDs pasted into `SoundController.CAR_SOUNDS`.
+
+## What the game needs (two loops)
+
+| Loop | Content | Length |
+|---|---|---|
+| **Idle** | Engine idling, steady. The rotary lope/brap at rest. | 5–10 s |
+| **Rev**  | Held at steady mid-high RPM (~4–6k), NOT a sweep. | 5–10 s |
+
+Steady-state matters: the game pitch-shifts and crossfades these, so a loop
+recorded during rising RPM will "wander" audibly. A long recording containing
+steady sections is fine — the good seconds get cut out in processing.
+
+## Step 1 — Record (or use existing video)
+
+Any source works: phone video, GoPro footage, a Zoom H5 recording. Existing
+videos of the car are fine — audio gets extracted. Best results:
+- Record OUTSIDE the car, mic a few feet from the exhaust.
+- Get ≥10 s of steady idle and ≥5 s of held revs.
+- Avoid wind over the mic and people talking.
+
+Drop the files on the NAS next to the reference photos:
+```
+unraid-1:/mnt/user/data/yy-roblox-games/0-fast-n-furious-sim/001-1993 Mazda RX-7/sound/
+```
+
+## Step 2 — Process into seamless loops (this workstation)
+
+Hermes does this — the box already has ffmpeg + numpy/scipy (`avsync-research`
+venv). Ask and it happens; for reference the recipe is:
+
+1. Extract audio: `ffmpeg -i clip.mp4 -vn -ac 1 -ar 48000 raw.wav`
+2. Find a steady window (visually via spectrogram, or scripted RMS/pitch scan).
+3. Cut the loop at zero-crossings and apply a short equal-power crossfade from
+   the tail onto the head so the seam is inaudible:
+   loop-body + (last ~150 ms faded out, overlaid on first ~150 ms faded in).
+4. Normalize to about -3 dBFS peak, export `.ogg` (smaller upload, Roblox-native):
+   `ffmpeg -i loop.wav -c:a libvorbis -q:a 6 rx7_idle_loop.ogg`
+5. Verify: play the file on repeat (`ffplay -loop 0`) — no click, no wander.
+
+Outputs: `rx7_idle_loop.ogg` + `rx7_rev_loop.ogg`, copied back to the NAS
+`sound/` folder and kept in `assets/audio/` locally (small enough to commit).
+
+## Step 3 — Upload in Studio (owner)
+
+1. Studio → **View → Asset Manager → Import** (or the Creator Dashboard →
+   Development Items → Audio) → upload both `.ogg` files.
+2. Audio uploads require an ID/phone-verified account; private audio uploaded
+   to YOUR account is playable in YOUR experiences — exactly our case.
+3. Copy each asset ID after moderation clears (usually minutes).
+
+## Step 4 — Wire in
+
+In `src/client/SoundController.luau`, uncomment and fill:
+```lua
+local CAR_SOUNDS: { [string]: EngineSounds } = {
+	rx7_fc = {
+		Idle = "rbxassetid://<idle id>",
+		Rev = "rbxassetid://<rev id>",
+	},
+}
+```
+Rebuild, playtest, adjust `IDLE_PITCH_*` / `REV_PITCH_*` / `CROSSFADE_*` to
+taste. Done — the RX-7 braps.
+
+## Why not a toolbox rotary?
+
+Searched; candidates exist (e.g. "13B ROTARY ENGINE SOUND", id 118059219867199)
+but can't be auditioned outside Roblox (asset downloads require auth), most are
+short unloopable rips, and rights are murky. The owner HAS the real car — its
+actual engine is strictly better than any stranger's rip. Toolbox is the
+fallback only if recording proves impractical: audition in Studio before
+committing to one.