## Summary
Replace per-instance `<style>` injection in `<hyperframes-player>` with a lazily constructed `CSSStyleSheet` adopted via `shadowRoot.adoptedStyleSheets`. One parsed stylesheet, many adopters — the studio thumbnail grid renders dozens of players concurrently and was paying for N parses of the same CSS.
## Why
Step `P1-1` of the player perf proposal. The previous implementation appended a `<style>` element to every shadow root, which means:
- N shadow roots → N copies of the same CSS string parsed into N independent style sheets.
- Each `<style>` lives in the DOM and contributes to layout/style invalidation work when its shadow root churns.
- The studio's project grid mounts ~30 players on initial load — that's 30 redundant parses of the same ~1 KB stylesheet on the critical path.
`adoptedStyleSheets` flips this: parse once at module load, hand the same `CSSStyleSheet` reference to every shadow root.
## What changed
- New `getSharedPlayerStyleSheet()` in `packages/player/src/styles.ts` — module-scoped and memoized; the sheet is built once per process and returned to every adopter.
- New `applyPlayerStyles(shadow)` is the single integration point. It **appends** (never replaces) the shared sheet so any pre-adopted sheets — host themes, scoped overrides, future caller-side injections — survive intact, and is idempotent so repeated calls don't multiply adoptions.
- SSR-safe via a `typeof CSSStyleSheet` guard. Failures (e.g. `replaceSync` throw, no constructor) are cached as `null` so we don't retry constructor failures forever.
- Defensive fallback path creates a per-instance `<style>` element when `adoptedStyleSheets` is unavailable (older runtimes, hostile environments). Behavior on those paths is unchanged from before.
- `PLAYER_STYLES`, `PLAY_ICON`, and `PAUSE_ICON` exports preserved — no public API change.
## Test plan
- [x] Unit tests in `styles.test.ts` cover sharing across instances, fallback when `CSSStyleSheet` is undefined or `replaceSync` throws, fallback when `adoptedStyleSheets` is unsupported on the shadow root, idempotency, and preservation of pre-existing adopted sheets.
- [x] Integration test in `hyperframes-player.test.ts` confirms two real `<hyperframes-player>` elements adopt the same `CSSStyleSheet` instance and inject zero `<style>` elements.
- [x] Build size delta is negligible (utility code replaces `container.appendChild` calls).
## Stack
Step `P1-1` of the player perf proposal. Followed by `P1-2` (scoping the media `MutationObserver`) and `P1-4` (coalescing parent media-time mirror writes) — all three target the studio multi-player render path.
