fix(core): treat unparsable timing attributes as missing instead of NaN

[calcarazgre646]

Problem

Element-level timing attributes go through parseFloat unguarded in the timing compiler and the HTML parser, so a typo like data-start="abc" propagates as NaN with no signal to the user:

• compileTag (timingCompiler.ts) computes data-end as start + parseFloat(durationStr) and serializes the result into the compiled HTML: the output literally contains data-end="NaN". The same unguarded reads exist in injectDurations, extractResolvedMedia (start/mediaStart; duration was already guarded), clampDurations, and the unresolved-div scan.
• parseHtml (htmlParser.ts) computes duration = Math.max(0, parseFloat(dataEnd) - start) — and Math.max(0, NaN) is NaN, so a garbage data-end lands in the timeline model as a NaN duration. updateElementInHtml writes data-end="NaN" back into the document when data-start is garbage.

Neither lint nor compile reports anything: the linter has no numeric check for element timing attributes, while composition-level duration is already validated (validateCompositionHtml errors on non-finite data-composition-duration, and extractCompositionMetadata guards with isFinite). Element timing attrs missed the same policy.

Change

Unparsable is treated as missing, which all of these paths already know how to handle:

• New parseTimingAttr(value, fallback) helper in timingCompiler.ts (the same policy extractResolvedMedia already applies to data-duration), used at every timing-attribute read in both files.
• compileTag additionally normalizes the compiled output: an unparsable data-start is rewritten to 0, and unparsable data-end/data-duration are stripped so the element takes the existing recompute-or-resolver path. Downstream consumers (parser, runtime) never see the garbage values.
• parseHtml falls back to its existing defaults (start 0, the 5s default duration).

No behavior change for valid inputs.

Tests

• Compiler: unparsable data-start normalized to 0 with data-end computed from it; unparsable data-duration dropped and the element marked unresolved; unparsable data-end stripped and recomputed; unparsable data-media-start resolved to 0; direct parseTimingAttr cases (including "Infinity"). All assert the compiled output contains no NaN.
• Parser: unparsable data-start parses as 0; unparsable data-end falls back to the 5s default instead of NaN; updateElementInHtml recomputes data-end from 0 instead of writing NaN.
• Full core suite: 1175 passed. bun run build green.