longcat: vendor the upstream source instead of cloning it (v0.2.52)

`longcat_video` — the pipeline, DiT, VAE, scheduler and INT8 quantisation helpers
— lives inside the upstream repo and is not on PyPI, so resolve_source_dir looked
for a checkout at ~/.coderai/LongCat-Video or a path baked at build time. Neither
exists in a published image, which means anyone pulling it had to clone from
GitHub before LongCat would run at all: a third-party network dependency in the
middle of someone else's install, on whatever `main` happened to be that day.

The 26 source files are now vendored under third_party/, MIT, pinned to commit
6b3f4b85. 496 KB, no weights, no wheels, nothing compiled. The resolver prefers
them over every fallback, and an explicit longcat_source still wins for anyone
running their own checkout.

They ship automatically: the image does COPY . /opt/coderai/app, and third_party
is not in .dockerignore — asserted by a test, since that is the sort of thing a
later ignore rule breaks silently.

The ISOLATED VENV is unaffected and still builds on first use from
requirements-longcat.txt. Only the source is vendored; torch 2.6+cu124 and the
rest are installed, not carried.

While confirming the fp8 question: upstream's quantization.py implements INT8
only — QuantizedLinear holds weight_int8 + weight_scale, and there is no fp8 code
anywhere in the package. The fp8 checkpoints on HF carry no loader either, and
szwagros/LongCat-Video-Avatar-1.5-fp8's own card is a verbatim copy of upstream's
and documents `--use_int8`, never fp8. So a VRAM-resident DiT goes through INT8,
not fp8.

88 tests.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0182DhR1eFNPQyGmDrpHbedY