v0.5.0

Audio: a per-receiver listening level, volume controls that can actually make
the K4's quiet stream loud, and — because this release began with a "no sound"
report that took hours to pin down — the diagnostics to tell you why there is
no sound, in one line instead of an afternoon.

Added

• Per-receiver volume and mute, beside each spectrum pane's A / B
  badge. Balances the two receivers in your headphones. Local to the app: it
  does not touch the radio's AF gain, so it changes nothing at the front panel
  or for anyone else connected. Mute keeps the level, so unmuting returns to
  where you had it, and mute always starts clear. (FR-RX-VOL-01)
• Audio diagnostics. The console now reports RX audio: N decoded / M played and a per-channel peak level in dBFS, which separates "no audio
  arriving", "arriving but cannot be played", and "playing, but the audio
  itself is silent" — three cases that previously looked identical. Failures to
  open the output device or build the decoder are reported with their actual
  error instead of being swallowed.
• Two diagnostic examples: cargo run -p k4-audio --example test_tone --features device plays a tone through the real playback path (1 kHz left =
  Main, 600 Hz right = Sub), which proves or clears the whole client side in
  seconds; --example list_audio prints the devices the app can see.

Changed

• Volume controls read 0–100 % and follow a perceptual (cubic) curve
  instead of being a raw multiplier. Unity sits near 40 % of the travel and the
  top reaches +24 dB, because the K4 can stream audio at around -45 dBFS —
  far too quiet for a noisy room at the old ceiling. Existing settings are
  migrated to the position that reproduces the same loudness, so upgrading does
  not change how loud your radio is.
• The per-pane VOL only attenuates; overall loudness is the master's job.
• Playback is clamped to the sample range, so the extra gain clips rather than
  distorting.
• The TX tag is gone from the transmitting spectrum pane — the accent
  border already says which pane transmits.

Fixed

• ESC in the diagnostics window closed the Settings dialog in the main
  window and left the log window open. Key presses were handled without regard
  to which window they came from.

Known: the K4 streams quietly, and AG does not change that

Measured on one K4: the streamed level sits around -45 dBFS, and AG
does not control it — neither the radio's front-panel knob nor AG sent over
the link changes the streamed level; both change the radio's own speaker
volume. Until a radio-side control is found, the app's own Volume is the
lever, which is why it now reaches +24 dB. Note that digital gain raises the
stream's noise along with the signal.

Two consequences worth knowing:

• Turning the radio's AF down to quiet the shack will not quiet your stream —
  and turning it up will not raise it. To listen remotely with a silent
  shack, switch the radio's internal speaker off (menu Speaker, Internal, or
  ME0001.0;) instead.
• If you hear nothing, the console's peak line now tells you whether the radio
  is sending sound at all.

---

Full history: CHANGELOG.md