fix(media): guard SynchronizedMedia.frameDomain before reading frameRate

[Mrxuba]

What

verifySynchronizedMedia dereferenced item.frameDomain.frameRate (and
item.frameDomain.frameCount) right after confirming only that the top-level
value is an object. When frameDomain is missing or is not an object, that
threw a bare TypeError: Cannot read properties of undefined (reading 'frameRate')
instead of a stated validation error.

This is the crash reported in #388, where reading an older / hand-edited
result.json whose SynchronizedMedia lacks a well-formed frameDomain
fails with exactly that message (@hypit/media@1#SynchronizedMedia ... Cannot read properties of undefined (reading 'frameRate')).

Fix

Add the same object() guard the sibling verifiers (verifyTimelineVisual,
verifyMuxedMedia, verifyRational, ...) already use, so a malformed frame
domain is reported as SynchronizedMedia.frameDomain must be an object.

   const item = object(value, "SynchronizedMedia") as unknown as SynchronizedMedia;
+  object(item.frameDomain, "SynchronizedMedia.frameDomain");
   verifyRational(item.frameDomain.frameRate, "SynchronizedMedia.frameDomain.frameRate");

Scope

This addresses only the unguarded frameRate crash from #388 (turning it into
a diagnosable validation error). It does not claim to make the bundled archive
replay end-to-end — that report also involves missing source.id / run.id
fields in the archived result, which are separate.

Tests

Adds packages/media/test/identity.test.ts:

• a well-formed SynchronizedMedia still verifies;
• a missing frameDomain and non-object frameDomain now throw the stated error.

pnpm --filter @hypit/media test (12 tests) and pnpm check both pass.