fix(core): root-cause id-less media wash in timingCompiler getAttr, drop the band-aid

[miguel-heygen]

Follow-up to #1790. That PR fixed the blank-wash/dropped-audio symptom with a producer-side band-aid (assignMissingMediaIds). This fixes the actual root cause and removes the band-aid.

Root cause

timingCompiler.ts getAttr built its regex with no name boundary at all:

tag.match(new RegExp(`${attr}=["']([^"']+)["']`))

So getAttr(tag, "id") matched the trailing id="…" inside data-hf-id="…" and returned a phantom. compileTag saw a "present" id and skipped its existing hf-video-N/hf-audio-N injection, leaving the element with no real el.id — which every render stage keys off (__render_frame_<id>__, runtime frame-swap, audio[id][src]). Hence the blank wash + dropped audio. Same bug family as the lint readAttr \b issue fixed in #1790.

Change

• getAttr → (?<![\w-]) lookbehind (one line). compileTag's auto-id injection now fires for data-hf-id-only media, in both the main composition and sub-compositions (parseSubCompositions runs the same compileTimingAttrs pass).
• Delete assignMissingMediaIds — redundant once the root is fixed (net −4 lines).
• Tests: timingCompiler unit tests for the video + audio id-injection boundary; regression fixture baseline regenerated on linux/amd64 (now hf-video-0 from compileTag).

Review findings (from post-merge review of #1790)

• 🟠 Twin getAttr boundary bug → fixed at root; band-aid removed.
• 🟡 Audio side untested → covered by a deterministic timingCompiler unit test (id-less <audio> → hf-audio-N). The render fixture stays video-only: a synthetic sine/noise audio track anti-aligns under the harness audio cross-correlation (deterministic −1.0); real audio content is unaffected, so a brittle audio render-baseline wasn't worth it.
• 🟡 Observability → intentionally not added: the band-aid is gone, compileTag auto-id is long-standing intended behavior, and the loud signal is the (now-fixed) media_missing_id lint rule.
• 🟡 studio_missing_editable_id warning still fires for id-less media — correct and unchanged; a real id remains the contract for Studio targeting / GSAP selectors. The compiler auto-id is a render safety net, not a license to omit id.