fix: complete FireRedASR2 (小红书) model download verification

[IchenDEV]

Outcome

Downloading the 小红书 FireRedASR2-AED local ASR model (mlx-community/FireRedASR2-AED-mlx) now completes: after a successful transfer Utter marks it .downloaded instead of model.asr_incomplete ("模型只下载了一部分…"), so the model becomes selectable and ready.

Root cause

ModelCatalog.asrRequiredFiles(for:) required tokenizer.json for FireRedASR2-AED, but that file does not exist in the upstream repository (manifest: config.json, cmvn.json, dict.txt, model.safetensors, train_bpe1000.model). The path-based completeness check could therefore never pass, even though the download itself succeeded.

SDLC bundle and risk

• Bundle: docs/sdlc/changes/2026-09-30-firered-asr-download-completeness/
• Risk: medium — model/runtime behavior
• Human decisions still required: PR approval/merge (intent and design approved in-chat 2026-09-30)

Verification

• bash scripts/sdlc-checks.sh
• bash scripts/ci-basic-checks.sh
• swift test — 794 tests, 18 skipped, 0 failures
• Regression test fails on the pre-fix list (3 assertion failures) and passes after
• Release-style app build (not run; no packaging change)
• Real-window QA (not run; no UI change)

Evidence: docs/sdlc/changes/2026-09-30-firered-asr-download-completeness/verification.md.

Residual risk and rollback

Mega-ASR-6bit still requires only ["config.json", "tokenizer_config.json"] (both exist upstream, so its download completes); tightening it is out of scope and tracked as residual risk. Rollback is a one-line revert of the required-file list.

Reviewer focus

Confirm the four required files match the upstream manifest and what the pinned mlx-audio-swift 0.1.3 FireRedASR2Model.fromDirectory actually reads.