chore(deps): pin diffusers to the commit that merged MiniMax H3

[Rockdu]

What

• Move diffusers from 0.38.0 to a git pin on the main commit that merged MiniMax H3 (f53d5520, huggingface/diffusers#14355).

Why

MiniMax H3 support is needed by #154, but no diffusers release carries it: v0.39.0 (2026-07-03) predates the merge (2026-08-05), and nothing has shipped since. A git pin is the only option until the next stable release.

Split out of #154 so the dependency upgrade is validated on its own. This is a two-minor jump (0.38.0 → post-0.39.0 main) and can break models that have nothing to do with H3 — that is exactly what this PR asks CI to answer, without the H3 training code in the way. requirements.txt here is byte-identical to #154's.

Upstream has four later commits touching H3 (SDNQ loading #14398, context parallel #14407, LoRA loading #14408, CUDA test fixes #14464). They are deliberately not picked up: pinning the merge commit keeps this identical to what #154 was validated against.

Files

File	Role
requirements.txt	the pin, plus a comment recording why it is a git ref and when to drop it

Checklist

• pre-commit run --all-files passes — not run locally
• Added/updated tests for new behaviour — none added; a dependency pin has no behaviour of its own, CI is the test
• pytest -x is green — not run locally (no torch in this environment); relying on CI
• If launch flags changed, python3 train.py --help still parses — no flags changed
• If a public flag was added, it appears in the CLI reference docs — no flags added
• If an example was added, it has a real walkthrough — no examples added

[Rockdu]

Merged with #190