fix(core,producer): resolve relative data-start id-refs in render compile

[valeriangalliat]

What

data-start="intro" (and intro + 0.5) already resolves in the live player. Render compile treated it as a number, so relative starts failed in actual renders.

Why

compileTimingAttrs / injectDurations wrote data-end from parseFloat("intro"). Extract prefers data-end over duration, so the clip's end was unusable.

parseSubCompositions did the same parseFloat on the host, so nested media inherited a broken offset.

How

• Use parseNumeric in the timing compiler. If start isn't numeric, leave data-end off so extract can resolve the id-ref.
• Use the existing engine resolveReferencedStart when reading sub-composition host start (same helper video/audio/image extract already use).

No new resolver. Regex compiler still has no DOM.

Test plan

• Unit tests added/updated
• Manual testing performed
• Documentation updated (if applicable)

---

• bunx vitest run src/compiler/timingCompiler.test.ts in packages/core
• bunx vitest run src/services/videoFrameExtractor.test.ts -t "still resolves relative data-start" in packages/engine
• bun test src/services/htmlCompiler.test.ts -t "resolves a host data-start" in packages/producer