Bump to v1.2.0: recording, sound cues, drift compensation, diagnostics

New user-facing features:

* Recording. Dedicated Record menu (Alt+O — moved from Alt+R to
  avoid clashing with the Receive audio checkbox), Start/Stop on
  Ctrl+R, settings dialog, per-profile source / format / bit-depth /
  channel-mode / folder. Three source modes (received only, sent
  only, both). Files are crash-resilient — a process crash
  mid-recording leaves a playable file containing everything up to
  the last header refresh (~5 seconds).

* Four output formats, all functional:
    - WAV: 16/24-bit PCM or 32-bit float, custom writer with
      periodic RIFF re-patching.
    - MP3: LAME 128–320 kbps CBR (via NAudio.Lame).
    - OGG-Opus: 96–256 kbps VBR (via Concentus.Oggfile, reusing the
      Concentus encoder from the wire path).
    - FLAC: 16/24-bit lossless (via CUETools.Codecs.FLAKE — pure
      managed, no native DLL).

* Recording start/stop sound cues. record start.wav and
  record stop.wav play around the recording transition. Played via
  System.Media.SoundPlayer to the default Windows output, separate
  from the recording pipeline so a normal recording does not contain
  the cue.

* Per-cue Preferences. The old single "Mute connect/disconnect
  sounds" checkbox is replaced by a CheckedListBox: Connect /
  Disconnect / Recording start / Recording stop. Old profiles with
  the legacy MuteConnectionCues=true are honoured on first load via
  a migration path in the new Load* helpers.

* Receiver-side drift compensation switched from discrete
  single-frame splices to a continuous WdlResampler at a smoothed
  rate ratio. SessionPlayout.cs rewrite.

Diagnostics (only active with Enable logs ticked):

* Per-stage discontinuity probes — sender raw capture (per backend,
  PushModeWasapi + Asio both wired), sender pre-encode (now per
  lane in BothIndependent, fixing a cross-stream artefact), receiver
  post-decode, post-ring, post-resampler.

* Wire-level packet sequence tracking on each PCM stream — in-order
  / missed / reordered / duplicated counts in the diag log.

* Clipped-sample delta in the diag log.

* New AudioStepProbe in RemSound.Core with per-channel scan helper.

UI changes:

* Record menu uses Alt+O (Rec&ord). Inside the menu, item mnemonics
  unchanged (S / T / O / C).

* Auto-tune interval combo label is mode-aware: "Auto-tune latency
  interval" in classic modes, "Auto-tune interval — WASAPI and ASIO"
  in BothIndependent. The combo's Enabled state now follows EITHER
  lane's auto-tune checkbox (was only the WASAPI one — bug).

Wire format and audio pipeline unchanged from v1.1 — v1.1 and v1.2
peers interoperate.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/RemSound.Sender/SenderLane.cs

@@ -52,6 +52,17 @@ internal sealed class SenderLane
     private OpusEncoderState opusEncoder;
     private int opusFrameStereoSamples;
 
+    // Per-lane pre-encode discontinuity probe. Moved here from AudioSender (2026-05-15) so
+    // each lane has its OWN probe state and the cross-buffer step measurement (which carries
+    // lastL/lastR across calls) only sees samples from one continuous audio stream. With the
+    // earlier shared-probe design, BothIndependent mode mixed two unrelated streams' samples
+    // into the same probe's cross-buffer carry, producing synthetic "steps" of arbitrary
+    // magnitude every time the two lanes' callbacks interleaved — making the diag log unable
+    // to tell a real capture glitch from instrumentation aliasing. Per-lane separation fixes
+    // that without changing what the probe measures.
+    private readonly AudioStepProbe preEncodeStepProbe = new();
+    public float TakeMaxPreEncodeStep() => preEncodeStepProbe.TakeMax();
+
     // Which render route this lane announces in its format packets. The receiver reads the
     // Lane byte on the wire and tags the matching SessionPlayout, which makes PlayoutEngine
     // route the lane's audio to the corresponding per-route IWaveProvider surface (lane
@@ -144,6 +155,20 @@ internal sealed class SenderLane
         var emitStart = diag ? System.Diagnostics.Stopwatch.GetTimestamp() : 0L;
         EnsureFormatPacketSent();
 
+        // Recording tap — the recorder gets the float audio about to be encoded. The lane
+        // doesn't know whether the recorder is running; the dispatcher early-outs when no
+        // callback is wired. Captured here (before encoding) so the recording is bit-clean
+        // float, independent of which codec the wire is using.
+        owner.DispatchSentSamples(stereoFloats);
+
+        // Discontinuity probe — what does the audio look like just before we encode it?
+        // Compared to the receiver's per-stage probes, this tells us whether artefacts are
+        // present at the sender side already (capture hardware glitch, mix-bus issue) or
+        // introduced somewhere in the wire / decode / playout chain. Per-lane probe — see
+        // <see cref="preEncodeStepProbe"/> field comment for why this isn't shared with the
+        // other lane in BothIndependent.
+        preEncodeStepProbe.ScanStereo(span);
+
         switch (owner.Codec)
         {
             case AudioTransportCodec.Pcm: