feat: align offline slide playback with live synth behavior

[ChuxiJ]

Summary

• share slide-note source resolution and slide scheduling between live transport playback and offline synth rendering
• make offline piano-roll render paths honor slide notes instead of rendering them as plain attack-release notes
• add focused node-environment tests for slide source resolution and shared slide scheduling behavior

Why

Before this PR, live playback respected isSlide notes via playSlideNote(...), but offline rendering ignored slides entirely. That meant export, freeze, bounce, and share could sound different from what users heard during playback. This change closes that parity gap.

Verification

• npx tsc --noEmit
• npx vitest run --environment node src/engine/__tests__/SynthEngine.test.ts
• npm run build

Notes

• This PR intentionally reuses the same slide helper from both useTransport and offlineRender so future glide changes stay in one place.

[ChuxiJ]

Triage: Hold — Stacked on synth improvement chain. Depends on #1019.

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