fix: remove hidden audio gain in renders (#362)

## Summary

This fixes a render-time audio correctness bug where Hyperframes applied a hidden post-mix gain to every rendered output, boosting audio by about +2.6 dB and causing clipping on normally leveled sources.

It also fixes a related mute bug where `data-volume="0"` was treated as falsy and silently converted back to full volume during audio track preparation.

Additionally, this PR fixes the Studio workspace typecheck path for `@hyperframes/player`, so local pre-commit/typecheck flows no longer depend on the Player package having been built first.

## Root Cause

The issue report measured a near-constant gain increase and suspected a hidden normalization step. After tracing the engine audio path, the root cause turned out to be explicit code, not FFmpeg behavior:

- `packages/engine/src/config.ts` defaulted `audioGain` to `1.35`
- `packages/engine/src/services/audioMixer.ts` always appended a post-mix FFmpeg filter:
  - `[mixed]volume=${masterOutputGain}[out]`
- with the default config, that meant every render got multiplied by `1.35`

That exactly matches the issue reporter's measured scalar boost.

While investigating the workaround, I also found a second correctness bug:

- `processCompositionAudio()` used `element.volume || 1.0`
- that coerced `0` to `1.0`
- so `data-volume="0"` did not actually mute the track in rendered output

Separately, the repo-level Studio typecheck could fail before any build step because:

- `packages/studio/src/player/components/Player.tsx` imports `@hyperframes/player`
- `packages/player/package.json` points TypeScript at built `dist/*` outputs
- in a fresh workspace, those built outputs may not exist yet
- Studio therefore failed type resolution for `@hyperframes/player` during pre-commit/typecheck

## What Changed

1. Set the engine default `audioGain` back to unity (`1`)
2. Preserve explicit zero volumes by changing `element.volume || 1.0` to `element.volume ?? 1.0`
3. Added regression coverage for both behaviors
4. Updated the producer-side config fixture to reflect the corrected default
5. Added a Studio tsconfig path mapping for `@hyperframes/player` to the local workspace source and widened `rootDir` so workspace typecheck succeeds without requiring a prior Player build

## Why These Changes Are Needed

This is not a UX preference issue; it is a correctness and API contract issue.

- The docs describe `data-volume` as a direct 0-1 control.
- Rendered output should preserve source levels unless the author explicitly changes them.
- Hidden global gain makes output non-deterministic from the author's perspective.
- `data-volume="0"` must mean silence, not full-volume playback.
- Local workspace typecheck should not require unrelated package build artifacts to exist first.

Leaving the current behavior in place means:

- voice recordings near normal peak levels can clip during render
- authors need undocumented manual compensation (`0.75`-ish scaling) to get unity output
- mute semantics in docs and code diverge
- local pre-commit/typecheck can fail for reasons unrelated to the actual diff being committed

## Testing

### Focused regression tests

Ran:

- `packages/engine/node_modules/.bin/vitest run packages/engine/src/config.test.ts packages/engine/src/services/audioMixer.test.ts`

Result:

- `10 passed`

These tests specifically verify:

- default resolved `audioGain` is `1`
- a track with `volume: 0` stays `volume=0` in the FFmpeg filter graph
- the post-mix output filter stays at unity gain (`[mixed]volume=1[out]`)

### Broader package verification

Ran:

- `bun run --filter @hyperframes/engine test`
- `bun run --filter @hyperframes/engine build`
- `packages/engine/node_modules/.bin/vitest run packages/producer/src/services/renderOrchestrator.test.ts`
- `bun run --filter @hyperframes/producer typecheck`
- `bun run --filter @hyperframes/studio typecheck`
- `bunx oxlint packages/engine/src/config.ts packages/engine/src/config.test.ts packages/engine/src/services/audioMixer.ts packages/engine/src/services/audioMixer.test.ts packages/producer/src/services/renderOrchestrator.test.ts`
- `bunx oxfmt packages/engine/src/config.ts packages/engine/src/config.test.ts packages/engine/src/services/audioMixer.ts packages/engine/src/services/audioMixer.test.ts packages/producer/src/services/renderOrchestrator.test.ts packages/studio/tsconfig.json`
- `bunx lefthook run pre-commit`

Results:

- full engine test suite passed (`309 passed`)
- engine build passed
- touched producer test file passed (`7 passed`)
- producer typecheck passed
- studio typecheck passed
- oxlint passed with `0 warnings, 0 errors`
- formatting passed
- pre-commit hook no longer hits the prior `@hyperframes/player` module-resolution blocker

## Known Verification Limitation

There is no meaningful browser UI flow for this bug: the defect is in the engine/CLI audio render pipeline rather than an interactive browser surface. Because of that, verification was done at the renderer and test level rather than through an agent-browser flow.

## User Impact

After this change:

- rendered audio matches source level by default
- authors no longer need to compensate for a hidden +2.6 dB boost
- `data-volume="0"` correctly mutes rendered audio
- the documented volume contract matches engine behavior again
- local workspace typecheck no longer depends on prebuilt `@hyperframes/player` artifacts

Closes #361.

packages/studio/tsconfig.json

@@ -16,6 +16,7 @@
     "declarationMap": true,
     "sourceMap": true,
     "outDir": "dist",
+    "rootDir": "..",
     "types": ["vite/client"],
     "lib": ["dom", "dom.iterable", "esnext"],
     "allowJs": true,