[ci][diagnostic]: capture GameCraft FA4 candidate on GB200

[SolitaryThinker]

Diagnostic only — do not merge.

This disposable branch generates one current-FA4 HunyuanGameCraft T2V candidate on GB200 and makes that small MP4 recoverable from the public Buildkite job log. It is separate from PR #1763 so build #5058 and its exact head remain untouched.

Safety gates:

• based on merged upstream/main at c9c558575804037ee95fd41014385a69bdbf7a85;
• runs only test_gamecraft_t2v_similarity, never I2V;
• pins FASTVIDEO_FA4=1, default-quality parameters, and bootstrap mode off;
• requires checkout SHA to equal BUILDKITE_COMMIT, exactly one visible GB200, and resolved FlashAttention 4;
• leaves the existing 0.93 SSIM threshold unchanged and requires a fresh matching SSIM JSON;
• performs no Hugging Face upload and needs no media-upload credential;
• forces lane exit 2, so it cannot be mistaken for a passing quality gate and does not trigger the exit-1 SSIM retry policy.

Pytest's noisy output is redirected to /tmp; only a bounded tail is printed. The generated MP4 is capped at 1 MiB and emitted last as numbered 3-KiB binary/base64 chunks between strict markers with byte count and SHA-256. The companion decoder accepts raw marker text or Buildkite's public { "output": ... } log JSON, refuses output overwrite, and verifies the complete chunk sequence, decoded size, and SHA-256.

Local validation:

• bash -n .buildkite/scripts/lanes/ssim.sh
• Python bytecode compilation of .buildkite/scripts/gamecraft_candidate_log.py
• pytest fastvideo/tests/contract/test_ci_test_collection.py -q (13 passed)
• targeted pre-commit (filename check passed; configured hooks skip these Buildkite paths)
• git diff --check
• exact round trip of the backed-up 447,050-byte canonical GB200 MP4 from raw and simulated timestamp-prefixed public Buildkite JSON with / converted to / and timestamp tags entity-escaped; both recovered SHA-256 29e8025d5a1495093ec7ccb150fc3d2eed1e13f1a67368a2aeacd6d52ae6e2e9; a deleted chunk was rejected
• entity-escaped simulated public JSON size: 654,626 bytes, 148 envelope lines, maximum rendered line length 4,594 bytes

After the candidate is decoded and visually reviewed, close this PR. Any canonical reference upload is a separate, explicitly user-authorized operation.

[SolitaryThinker]

/test ssim

[mergify]

Merge Protections

🔴 1 of 1 protections blocking · waiting on 👀 reviews and 🤖 CI

	Protection	Waiting on
🔴	PR merge requirements	👀 reviews and 🤖 CI

🔴 PR merge requirements

Waiting for

• #approved-reviews-by>=1
• check-success=full-suite-passed
• check-success~=pre-commit

This rule is failing.

• #approved-reviews-by>=1
• check-success=full-suite-passed
• check-success~=pre-commit
• check-success=fastcheck-passed
• title~=(?i)^\[(feat|feature|bugfix|fix|refactor|perf|ci|doc|docs|misc|chore|kernel|new.?model|skill|skills|infra)\]

[SolitaryThinker]

/test ssim

[SolitaryThinker]

/test ssim

[SolitaryThinker]

Rebased onto current main so the reference candidate uses the current runtime path; no content changes (range-diff clean).

[SolitaryThinker]

/test ssim