New alternative local player backend gated behind the `localav` build tag.
Build with: go build -tags "migrated_fynedo localav" ./...

Stack:
- libavformat/libavcodec/libavfilter for demux, decode, and DSP
- miniaudio (vendored single-header 0.11.21) for audio output and
  device enumeration via CoreAudio/WASAPI/ALSA

Key features:
- Gapless playback via double-slot decoder: next track is pre-opened and
  swapped in immediately when the current decoder hits EOF, before the
  ring buffer drains, eliminating audible gaps
- SPSC lock-free ring buffer between decode goroutine and miniaudio callback
- 15-band parametric EQ via avfilter graph (same filter string as mpv backend)
- ReplayGain from file tags via avfilter volume node
- Audio device enumeration and selection including exclusive mode
- Peak/RMS metering via astats avfilter
- Seek safety: decode goroutine is stopped synchronously before
  av_player_seek rebuilds the filter graph (prevents SIGSEGV race)
- Direct PCM sample access hook point for future ProjectM visualizer

Files:
- backend/player/localav/av_player.{h,c} — C engine
- backend/player/localav/player.go — CGo bindings, implements LocalPlayer
- backend/player/localav/miniaudio.h — vendored miniaudio 0.11.21
- backend/player/localav/cgo_{darwin,linux,windows}.go — platform CGo flags
- backend/app_player_localav.go — //go:build localav app init

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Replace decode-ahead time tracking with frame-counter position formula
(position_offset + (frames_played_total - position_clock_ref) / sample_rate)
so the UI position reflects actual audio output, not the 4-second decode
lookahead in the ring buffer.

Move peak meter computation from the astats AVFilter (decode-ahead path)
into the miniaudio callback (actual playback path) so peaks are in sync
with audible audio. This also eliminates the SIGSEGV-prone filter graph
rebuild that av_player_set_peaks_enabled previously triggered.

Fix double mutex unlock in gapless swap (decoder_lock released inside the
if-block then again unconditionally), which is UB on non-recursive
pthread_mutex and could cause the decode loop to exit permanently.

Fix gapless track-change threshold to use frames_played_total + ring.fill
instead of frames_written_total, which silently drifts after ring_clear
in av_player_seek.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

EQ bands and preamp are now applied via ma_peak2 biquad filters in the
miniaudio callback rather than FFmpeg's filter graph. This gives immediate
EQ response without waiting for the ~4s ring buffer to drain, and ensures
the peak meter reflects raw pre-EQ decoded audio as intended.

Go now passes numeric band params (frequency, gain_db, Q) to C directly
instead of building FFmpeg avfilter strings. A double-buffered eq_bank_t
with an atomic index swap keeps the audio callback lock-free.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Use a frame-position-keyed bitrate history ring to delay the displayed
bitrate by the ring buffer depth (~4s), so the UI shows the bitrate
matching the audio currently playing rather than the most recently
decoded packet. Also fixes post-seek desync by realigning
frames_written_total to frames_played_total on seek.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Move ReplayGain from FFmpeg filter graph (decode stage) to the miniaudio
output callback, matching the EQ pattern. The decoder reads all four RG
tags (TRACK/ALBUM GAIN/PEAK) from file metadata at open time. A linear
gain multiplier is computed from the mode, prevent-clip flag, and preamp
offset, then applied in the callback after EQ and before master volume.
For gapless playback, a pending gain switch is armed at the track
boundary so the new track's gain takes effect exactly when its audio
starts playing through the ring buffer.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

…new track

av_player_open and av_player_stop freed next_dec without holding
decoder_lock, racing with av_player_open_next which acquires the lock.
When the time-pos polling goroutine called SetNextFile concurrently with
a user-initiated PlayFile, both could free the same decoder.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Add av_waveform.c implementing av_analyze_waveform() directly via FFmpeg,
replacing the MPV transcode-to-WAV pipeline. Split StartWaveformGeneration
into build-tag-gated files (waveformimage_localav.go / waveformimage_mpv.go)
so both backends are fully supported. Analysis supports cooperative
cancellation via an atomic cancel flag, wired to the job context so
seeking/navigation stops in-flight decodes promptly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>