M10.5: Text-based editing / transcripts

- crates/speech (L2): Transcriber trait, FixedTranscriber, Whisper model catalogue (pinned,
  SHA-256 verified) with optional downloader (feature `download`: ureq + rustls + RustCrypto),
  pure-Rust Whisper inference on candle with word timestamps (feature `whisper`), log-mel, VAD
  word tightening, MFCC speaker clustering. Both features off by default and for wasm.
- project: per-media-item transcripts (Project::transcripts); schema v9 with a no-op v8->v9
  migration and a v8 fixture.
- edit::transcript: sequence transcript derived from clip transcripts, search, paragraphs,
  word ranges, pause/filler detection, ripple delete of many ranges, caption layout.
- engine: transcript.* commands (generate, set, delete, inspect, search, select, extract, lift,
  renameSpeaker, removeFillers, removePauses, createCaptions, models, downloadModel);
  Session::transcriber for hosts/tests; features `whisper` / `speech-download` (also on the
  desktop and CLI apps). Without whisper, transcript.generate fails with a clear error.
- ui: Text panel Transcript tab (Transcribe button, speaker paragraphs of clickable words,
  Shift+click selection -> In/Out, extract/lift/remove fillers/create captions, search) with
  text.transcript.* automation ids; headless UI test.
- docs/transcripts.md, crates/speech/README.md, architecture and project-files updates.
  whisper-tiny measures 12.2% WER over 797 words on local samples.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/project-files.md

@@ -8,7 +8,7 @@ writing them is `crates/format` (`filmcraft-format`); the engine's `file.*` comm
 ```json
 {
   "format": "filmcraft.project",
-  "schema_version": 8,
+  "schema_version": 9,
   "generator": "FilmCraft 0.1.0",
   "project": { "name": "…", "settings": { … }, "root": { … }, "items": { … }, "next_id": 48 }
 }
@@ -32,6 +32,7 @@ writing them is `crates/format` (`filmcraft-format`); the engine's `file.*` comm
 | 6 | M11.7/M11.9 | media identity fingerprints, ingest settings; no-op step |
 | 7 | M5.5 | effect masks: `masks` on effect instances with keyframable Bézier `Path` values; no-op step |
 | 8 | M12 | multi-camera source sequences and clips, merged clips; no-op step |
+| 9 | M10.5 | transcripts of media items (`project.transcripts`); no-op step |
 
 ### Migrations
 
@@ -59,8 +60,8 @@ on disk is unchanged until you save. The **first save over it** keeps the origin
 
 A file with a `schema_version` newer than the build supports is **refused**, not half-read:
 
-> this project was saved by a newer version of FilmCraft (project schema v9); this build reads up to
-> v8. Update FilmCraft to open it.
+> this project was saved by a newer version of FilmCraft (project schema v10); this build reads up to
+> v9. Update FilmCraft to open it.
 
 Reading it partially and saving it back would silently drop whatever the newer version added.