A cache dir's completion marker only proves the save FINISHED, not that every
file landed intact. A transient truncated write — repeatedly a 0-byte
tokenizer/tokenizer_config.json — slipped into an otherwise "valid" cache and
then threw JSONDecodeError on EVERY subsequent video load, knocking the pipeline
off its fast path into the offload fallback ladder (balanced→sequential→disk),
which churned for hours, leaked ~22 GB VRAM, and died on a meta-tensor error.

Add _first_bad_json(dir): walk the (small) JSONs and flag the first that is
empty or unparseable — the big weights are .safetensors and aren't scanned, so
it's cheap. Wire it in on BOTH sides:
 - load: valid() and component_valid() now invalidate + return False when any
   cached JSON is corrupt, so a poisoned cache becomes a clean rebuild instead
   of a death spiral.
 - save: save()/save_component() verify the temp dir before committing, and
   mark_monolithic_complete() refuses to finalize a dir with a corrupt JSON —
   so a truncated write is never cached in the first place.
Added invalidate_path(p) helper.

Verified: _first_bad_json flags 0-byte and garbage JSONs, passes clean ones.
The already-poisoned Wan2.2-VACE cache was deleted out-of-band to unblock.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RdMufYvtTbtGDWsiZVoXce