v0.14.0

Added

• Runtime-adjustable per-output-track gain: Each output track (a track_mappings key) can
  now carry a gain in dB via an optional track_gains map in the profile audio section
  (-60 to +12 dB; values at or below -60 mute). Gains are adjustable live from the web UI
  (per-track slider in the Tracks card), gRPC (SetTrackGain / GetTrackGains), and OSC
  (/mtrack/track/*/gain, with per-track feedback in the broadcast loop for motorized
  faders). Gain state is lock-free and changes ramp across one audio batch to avoid zipper
  noise, with a constant-gain fast path that keeps the mixer inner loop at a single
  load + FMA per mapping edge (criterion benchmark added in benches/mixer.rs). Runtime
  adjustments persist to the active profile after a short debounce — in both inline-profile
  and profiles_dir layouts — so they survive restarts.

Changed

• Configs are validated at load: the player config and profile files now run semantic
  validation when loaded, so errors like a missing audio device, channel 0 in
  track_mappings, or out-of-range track_gains fail fast at startup with readable
  messages instead of surfacing as endless device-retry loops or cryptic playback errors.

Fixed

• First-profile setup in the web UI: creating the first hardware profile through the
  config editor was nearly impossible. Switching to another section (e.g. track mappings)
  before the first save silently turned the new profile into an update of a nonexistent
  backend profile ("Invalid profile index 0 (have 0 profiles)"); saving without track
  mappings failed with a raw "missing field track_mappings" parse error; and saving
  without a device produced an equally cryptic "missing field device". New profiles now
  stay new across section switches and back-navigation, track_mappings is optional while
  a profile is being set up (playing a song with no mappings gives a descriptive error
  instead), a missing device reports "audio device must not be empty", file-based profile
  saves (profiles_dir layouts) are validated server-side before being written, and the
  web UI now surfaces the backend's full validation error list on save failures.