Router warnings never reach the user without --explain

[kiki830621]

What

Every warning the router produces — including "requested backend 'X' is unavailable; selecting automatically" — is folded into the explanation string and printed only when --explain is passed.

Sources/bestasr/BestASRCommand.swift:143-146:

print("Wrote \(result.format) transcript to \(result.outputPath)")
if explain {
    FileHandle.standardError.write(Data((result.explanation + "\n").utf8))
}

CommandCore.swift:447 is the only place rec.warnings is consumed, and it appends into that same explanation.

Why it matters

The user asks for one backend and silently receives another's output. Observed during #121 verification:

$ bestasr transcribe cv-zhtw-2.wav --backend apple-speech --language zh
Wrote txt transcript to …/cv-zhtw-2.txt          ← that is the entire output

The transcript was produced by mlx-audio Whisper, not apple-speech. Only --explain revealed it. Anyone comparing backends by hand would have recorded one backend's numbers under another's name.

The same suppression hides the other router warnings, several of which exist precisely to stop a silent quality loss:

• "model 'X' does not list support for language 'Y' — output quality is not established" (recommend/transcribe 的 --language auto 未偵測音訊語言，非英文內容被推薦英文專用後端 #105 declared-language gate)
• the ColdStartPrior.ensureFits memory-downgrade warnings
• the unverified-model warning (中文 high-value ASR 家族評估 — FluidAudio 已內建 Paraformer/SenseVoice（零新依賴，優先於 MLX-Swift Qwen3-ASR）(follow-up from #35) #50)

Each was written to be seen. None is, by default.

Scope

Pre-existing, not introduced by #121 — git log -L143,146:Sources/bestasr/BestASRCommand.swift traces it to the original CLI commit (471218a, 2026-07-02), and main is byte-identical there. It affects every backend, not just the new one.

Suggested direction

Separate reasons from warnings: reason is explanatory detail and belongs behind --explain; warnings are things the user needs to know to trust the output, and belong on stderr unconditionally (a --quiet can opt out). Both already exist as separate arrays on ASRRecommendation, so the split is available without a schema change.

Worth checking benchmark and recommend for the same pattern.

Acceptance

• transcribe --backend <unavailable> prints the substitution warning without --explain
• --explain output is unchanged
• a test asserts the warning reaches stderr on the default path — the current suite would not have caught this

[kiki830621]

Verify (via PR #141)

Result: FAIL — 2 blocking, 6 follow-up. Engine: 4 lens Agents + sequenced Devil's Advocate (opus) + Codex gpt-5.6-sol — 6 independent verifiers, two model families. Diff-freshness gate PASS at 4f9f2dd.

Full report: #141 (comment)

Acceptance coverage for this issue: A1 FULLY, A2 FULLY (untested), A3 PARTIALLY — no test exercises the CLI printing path, so deleting BestASRCommand.swift:156-165 leaves all 447 tests green. That is the failure mode A3's own purpose clause names, reproduced one layer up.

Second blocking item: the #50 unverified-model notice this issue names explicitly is still reasons.append at Router.swift:210-212, while CHANGELOG.md:131-133 states it was unhidden.

See the "#141" report for the merged findings table, the adversarial pass (which partially overturned a 4-source consensus on the quality-floor notice — it belongs in reasons by spec), and the recorded failed attacks.