feat(captions): whisper transcription and SRT/VTT import

.claude/skills/rustmotion/rules/captions-workflow.md

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+# Rule: Captions Workflow (audio → word timings → caption component)
+
+Word-synced captions are a two-step pipeline: generate word timings with
+`rustmotion captions`, then inject them into a scenario through the variables
+system. Never hand-write word timings for real voiceovers.
+
+## Step 1 — Generate word timings
+
+```bash
+# From audio (requires whisper.cpp: brew install whisper-cpp)
+rustmotion captions voice.mp3 -o words.json
+
+# Pick a model / force the language
+rustmotion captions voice.mp3 -o words.json --model small --lang fr
+
+# Offline import from existing subtitles (no whisper needed)
+rustmotion captions --from-srt subs.srt -o words.json
+rustmotion captions --from-vtt subs.vtt -o words.json
+```
+
+Output shape (also valid as a `--props` file, on purpose):
+
+```json
+{
+  "words": [
+    { "text": "Hello", "start": 0.0, "end": 0.32 },
+    { "text": "world", "start": 0.32, "end": 0.7 }
+  ]
+}
+```
+
+- Transcription uses a whisper.cpp binary found in PATH (`whisper-cli`,
+  `whisper-cpp`, `main`). Models are resolved from
+  `~/.cache/whisper/ggml-<name>.bin` or a direct `.bin` path.
+- **SRT/VTT approximation**: subtitle cues carry sentence-level timing only,
+  so each cue's duration is spread **uniformly** across its words. Good
+  enough for karaoke modes; for precise word pops prefer real transcription.
+
+## Step 2 — Inject into the scenario
+
+Declare a `words` variable in `config` and reference it from the caption
+component:
+
+```json
+{
+  "config": { "words": { "type": "array", "default": [] } },
+  "scenes": [
+    {
+      "duration": 5,
+      "children": [
+        {
+          "type": "caption",
+          "mode": "word_pop",
+          "words": "$words",
+          "active_color": "#FFFFFF",
+          "pill_color": "#FF3366",
+          "position": { "x": 0, "y": 1600 },
+          "style": { "width": "100%", "font-size": 72 }
+        }
+      ]
+    }
+  ]
+}
+```
+
+```bash
+rustmotion render -f scenario.json --props words.json
+```
+
+## Caption modes
+
+| Mode | Behavior |
+|---|---|
+| `highlight` (default) | All words visible, active word takes `active_color` |
+| `karaoke` | Same as highlight (karaoke-style progression) |
+| `word_by_word` | Only the active word, centered, no pill |
+| `word_pop` | TikTok style: only the active word, spring scale-in, pill background |
+| `karaoke_pop` | Full line visible + active word scales 1.15x with `active_color` and pill |
+
+- `pill_color` (word_pop / karaoke_pop): pill background behind the active
+  word. Defaults to black at 70% (`#000000B3`).
+- `max_width`: wraps the line in highlight/karaoke/karaoke_pop modes — always
+  set it on vertical formats to avoid viewport overflow.
+- The caption paints from its baseline: position it with enough headroom
+  (font-size + pill padding above the anchor point).