…tsx split (#144)

## Summary

Full code quality review of the studio package, fixing 22 of 25 findings. Removes dead code, extracts modules from App.tsx, fixes accessibility and performance issues.

## Critical fixes (3)

- **`aria-valuenow`** on seek bar now updates imperatively via `liveTime.subscribe` — screen readers previously always reported position 0
- **Speed menu** closes on outside click (was permanently stuck open)
- **RenderQueue auto-scroll** moved from render phase to `useEffect` (was violating React render purity via `queueMicrotask` during render)

## Dead code removed (-331 lines)

| File | Lines | Why dead |
|---|---|---|
| `PreviewPanel.tsx` | 180 | Replaced by NLELayout + NLEPreview |
| `useCodeEditor.ts` | 80 | Exported but never imported |
| `formatTick` alias | 2 | Deprecated, unused |
| `onClipChange` prop | 5 | Declared, never used |
| `trackH` prop | 5 | Declared, never used |
| `editRange*` + updaters in store | 60 | Never read or written |

## App.tsx extraction

| Extracted to | Lines | What |
|---|---|---|
| `components/LintModal.tsx` | 130 | Lint results modal + LintFinding type |
| `components/MediaPreview.tsx` | 75 | Image/video/audio/font file previewer |
| `utils/mediaTypes.ts` | 15 | Shared regex constants (App.tsx and AssetsTab.tsx had diverged copies) |

## Performance fixes

- `useMemo` for `compositions`/`assets` derivation from `fileTree`
- `useMemo` for `buildTree(files)` in FileTree
- Debounced `handleContentChange` PUT (600ms — was firing on every keystroke)
- CompositionsTab iframe hover debounced (300ms — was mounting immediately)
- `VideoFrameThumbnail` re-extracts frame when `src` prop changes

## Not addressed (3 — low priority)

- #6: SystemIcons consolidation (large refactor across many files)
- #16-17: Overlay dismiss pattern standardization
- #18: Inline SVG → Phosphor replacement (gradual, per-PR)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

## Summary

Adds critical rendering constraints to the `hyperframes` skill discovered from eval analysis of 27 agent-generated compositions. These guardrails prevent agents from producing compositions that technically work but render poorly.

## What it fixes

| Rule Added | Eval Prompts Affected | Issue |
| --- | --- | --- |
| Ban `repeat: -1` | #20 loading-spinner (2.0/5) | Infinite timeline broke capture engine |
| Ban async timeline construction | #16 particle-logo (2.6/5) | Timeline empty at capture time |
| Min font size 16px (labels), 20px (body) | #7, #8, #13, #14, #15, #19 | Illegible text after encoding |
| Ban full-screen dark linear gradients | #3, #5, #10, #14 | H.264 color banding |
| `<link>` fonts over CSS `@import` | #7, #24 | Font loading race conditions |

## Changes

- **Rules section**: Added `repeat: -1` ban, async timeline ban, items 8-9 to "Never do" list
- **Typography section**: Expanded font size guidance with specific minimums per text role (headlines, body, labels)
- **New "Backgrounds and Color" section**: Guidance on avoiding gradient banding
- **Output Checklist**: 5 new items covering all new constraints

## Test plan

- [ ] Run eval with updated skill and compare avg quality scores
- [x] Skill renders correctly in `/hyperframes` invocation