refactor: draw the src/scripts boundary, drop dead files

Make each directory answerable by path alone: src/ is what the Remotion build
compiles, scripts/ is what you run by hand.

Removed:
- `ffmpeg.sh` — never worked; every parameter was assigned `$1`, so bake/bypass
  were broken from the start. Superseded by `scripts/ffmpeg_tools.py bake|still`.
- `src/lrc2srt.ts` — the TypeScript port; `scripts/lrc2srt.py` is the script that
  was actually maintained, and it now handles the multi-timestamp cases the TS
  version did.
- `scripts/bom-gen.ts` — superseded by `scripts/bom_gen.py`.
- `scripts/ffmpegusage.txt` — legacy command notebook. Every recipe in it is now
  reproduced by `ffmpeg_tools.py`, including the `-map 0:v:0 -map 1:a:0 -c:v copy`
  mux and the FLAC `-compression_level 12` case, so it has no unique content left.

Also drops the `bom-gen` alias from the dispatcher (dash-case was TypeScript
naming) and documents the boundary in README.md and REMOTION-QUICKSTART.md.

`npm run lint` (eslint src && tsc) passes with src/ reduced to Remotion input.

REMOTION-QUICKSTART.md

@@ -42,17 +42,25 @@ The Studio auto-reloads on file changes. No manual refresh needed.
 
 ```
 .
-├── src/
+├── src/                  # Remotion input — compiled by tsc/Remotion
 │   ├── index.ts          # Entry point — calls registerRoot()
 │   ├── Root.tsx          # React root component
-│   ├── Composition.tsx   # Composition definitions (placeholder)
-│   └── lrc2srt.ts        # LRC → SRT subtitle converter
+│   └── Composition.tsx   # Composition definitions (placeholder)
+├── scripts/              # Operator tooling — Python, stdlib only
+│   ├── tools.py          # Dispatcher: python3 scripts/tools.py --help
+│   ├── ffmpeg_tools.py   # mux / bake / still / strip / probe
+│   ├── lrc2srt.py        # LRC → SRT
+│   ├── srt2ass.py        # SRT → ASS (the step before baking)
+│   ├── bom_gen.py        # Bill of materials for untracked assets
+│   └── hwprobe.py        # What this machine can actually encode
 ├── package.json          # Remotion + React + Tailwind deps
 ├── remotion.config.ts    # CLI config (rspack, jpeg output format)
-├── tsconfig.json         # TypeScript (strict, ES2018)
-└── ffmpeg.sh             # Helper for ASS subtitle baking
+└── tsconfig.json         # TypeScript (strict, ES2018)
 ```
 
+`src/` and `scripts/` do not mix: the former is what the video build compiles, the
+latter is what you run by hand.
+
 ## Compositions
 
 Compositions are defined in `src/Composition.tsx`. Each has: