test(generate): lock in sync(2) void-but-returns classification

Audit of bare sync(2) per man 2 sync: void sync(void) takes no args and
returns no value. Confirmed it is correctly classified KindNull in
FamilyFS, its ret is UNCLASSIFIED, and — unlike noreturn exit/exit_group —
its exit handler IS emitted because sync does return (void != noreturn).
Docs and generated maps already match; no code or doc changes needed.

Add lock-in tests:
- TestGenerateSyncHandler: enter null_event with no arg capture (sync has
  no args at all), live exit handler emitted, ret recorded UNCLASSIFIED.
- TestClassifyRetSyncUnclassified: meaningless void ret stays UNCLASSIFIED.
- TestSyncIsNotNoreturn: guards sync from the noreturn suppression list.
- Add sync (FamilyFS) to the family/exit-handler table test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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