feat(scripts): unified Python toolchain, stdlib only

Replaces the scattered TypeScript/shell helpers with one Python toolchain that
runs as plain files (`python3 scripts/tools.py <tool> ...`) with no third-party
dependencies.

New tools
- `_common.py`     shared plumbing (Report, run, ffmpeg discovery, argparse ergonomics)
- `hwprobe.py`     what this machine can actually encode (software + hardware)
- `srt2ass.py`     SRT -> ASS, the missing step between lrc2srt and baking
- `ffmpeg_tools.py` mux / bake / still / strip / probe, with hardware encode
- `bom_gen.py`     BOM generator, ported from scripts/bom-gen.ts
- `tools.py`       dispatcher so any tool is one command away
- `README.md`      usage for all of the above

Rewritten
- `lrc2srt.py` now handles multiple timestamps per line, timestamp-less cue lines
  and blank/comment lines, and survives malformed input. The legacy version
  emitted an empty cue plus a stray cue for every line.

Hardware encoding
This host is AMD (Strix Halo, VCN 4.0.5), so nvenc in the legacy notes is
reference only. `hwprobe.py` probes and reports what genuinely works here:
VA-API via /dev/dri/renderD128 gives h264_vaapi, hevc_vaapi and av1_vaapi.
`ffmpeg_tools.py --video auto` picks the hardware encoder when the render node
is usable and falls back to libx264/libx265 when it is not.

Two bugs found and fixed while wiring this up:
- the VA-API capability probe fed frames straight to the encoder without
  `format=nv12,hwupload`, so ffmpeg failed with "Function not implemented" and
  the toolchain silently fell back to software on a machine where hardware
  works. VA-API encoders only accept GPU-resident frames; the probe now uploads.
- `bake --video copy` only failed when the copied stream happened to be VA-API.
  Burning subtitles always changes pixels, so it now refuses `copy` outright
  with a clear message.

Also makes -q/-n usable on either side of the mode word, and ignores
__pycache__ in .gitignore.

.gitignore

@@ -36,3 +36,7 @@ node_modules
 dist
 out
 .env
+# Python (helper scripts)
+__pycache__/
+*.py[cod]
+.venv/