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* fix(transcoding): clamp target channels to codec limit (#5336) When transcoding a multi-channel source (e.g. 6-channel FLAC) to MP3, the decider passed the source channel count through to ffmpeg unchanged. The default MP3 command path then emitted `-ac 6`, and the template path injected `-ac 6` after the template's own `-ac 2`, causing ffmpeg to honor the last occurrence and fail with exit code 234 since libmp3lame only supports up to 2 channels. Introduce `codecMaxChannels()` in core/stream/codec.go (mp3→2, opus→8), mirroring the existing `codecMaxSampleRate` pattern, and apply the clamp in `computeTranscodedStream` right after the sample-rate clamps. Also fix a pre-existing ordering bug where the profile's MaxAudioChannels check compared against src.Channels rather than ts.Channels, which would have let a looser profile setting raise the codec-clamped value back up. Comparing against the already-clamped ts.Channels makes profile limits strictly narrowing, which matches how the sample-rate block already behaves. The ffmpeg buildTemplateArgs comment is refreshed to point at the new upstream clamp, since the flags it injects are now always codec-safe. Adds unit tests for codecMaxChannels and four decider scenarios covering the literal issue repro (6-ch FLAC→MP3 clamps to 2), a stricter profile limit winning over the codec clamp, a looser profile limit leaving the codec clamp intact, and a codec with no hard limit (AAC) passing 6 channels through. * test(e2e): pin codec channel clamp at the Subsonic API surface (#5336) Add a 6-channel FLAC fixture to the e2e test suite and use it to assert the codec channel clamp end-to-end on both Subsonic streaming endpoints: - getTranscodeDecision (mp3OnlyClient, no MaxAudioChannels in profile): expects TranscodeStream.AudioChannels == 2 for the 6-channel source. This exercises the new codecMaxChannels() helper through the OpenSubsonic decision endpoint, with no profile-level channel limit masking the bug. - /rest/stream (legacy): requests format=mp3 against the multichannel fixture and asserts streamerSpy.LastRequest.Channels == 2, confirming the clamp propagates through ResolveRequest into the stream.Request that the streamer receives. The fixture is metadata-only (channels: 6 plumbed via the existing storagetest.File helper) — no real audio bytes required, since the e2e suite uses a spy streamer rather than invoking ffmpeg. Bumps the empty-query search3 song count expectation from 13 to 14 to account for the new fixture. * test(decider): clarify codec-clamp comment terminology Distinguish "transcoding profile MaxAudioChannels" (Profile.MaxAudioChannels field) from "LimitationAudioChannels" (CodecProfile rule constant). The regression test bypasses the former, not the latter.