* fix(engine): resolve relative data-start references in video-frame extraction
<video data-start="intro"> (a relative reference to another clip's end) is
resolved by the browser runtime but parseVideoElements/parseImageElements did a
raw parseFloat, yielding NaN start/end. The FrameLookupTable active-window
checks (start <= t <= end) are then always false, so the clip is never injected
and composites BLANK in the final render — while lint/validate/inspect/snapshot
and the live preview all look fine. The docs' Relative Timing section teaches
exactly this pattern on <video>.
Share the pure reference-syntax parser (parseStartExpression) out of the runtime
resolver into @hyperframes/core, and resolve references in the extractor against
the linkedom document it already holds: a reference resolves to the target
clip's resolved start + its duration (data-duration or data-end) + offset,
mirroring the runtime. Cycle-guarded; an unknown target or unknown duration
falls back to the target's start / 0 (never NaN), matching runtime semantics.
Natural-media-duration-only targets aren't known at parse time (same limit as
the runtime's fallback). parseImageElements gets the same fix.
Runtime resolver behavior is unchanged (its 25-case suite still passes).
* chore: re-trigger CI to refresh a stuck CodeQL aggregate check
