feat: align offline renders with canonical instruments

[ChuxiJ]

Summary

• align offline piano-roll rendering with canonical track.instrument state instead of relying on legacy synthPreset mirrors
• reuse the same synth playback chain for live and offline rendering so FM and subtractive runtime settings stay in parity
• cover the new canonical render-source behavior with focused export, freeze, and helper tests

Details

• export a shared createSynthPlaybackChain(...) from SynthEngine and reuse it in offlineRender
• let offline MIDI rendering consume canonical FM/subtractive instrument payloads, including the modulation rack introduced in the previous PR
• add getTrackInstrumentPlaybackSource(...) and getTrackSamplerPlaybackState(...) helpers to normalize piano-roll render inputs across callers
• update exportMix, bounceInPlace, freezeTrack, and projectSharingService to use the canonical playback helpers
• add a focused exportMix test file plus helper/freeze-service assertions for canonical FM and sampler paths

Validation

• npx tsc --noEmit
• npx vitest run --environment node src/engine/__tests__/SynthEngine.test.ts src/utils/__tests__/trackInstrument.test.ts src/engine/__tests__/exportMix.test.ts src/services/__tests__/freezeTrackService.test.ts
• npm run build
• curl --noproxy '*' -I http://127.0.0.1:4175/ returned 200 OK during local smoke after starting the dev server

Notes

• This PR is stacked on top of feat: add synth runtime modulation rack #1018.
• Browser automation smoke was limited by the current local Playwright/Chrome persistent-session issue, so validation focused on typecheck, node-environment tests, build, and local dev-server reachability.

[ChuxiJ]

Triage: Stacked draft PR — This is stacked on #1018 (synth modulation runtime). Both are drafts. These form a synth improvement chain (#1015 → #1018 → #1019 → #1060). Recommend: review and merge the full chain together once the base (#1015) is rebased and validated.

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