Bump to v3.0.0: Opus live latency, codec rename, profile-resume after update, save-through-lock

* New "Opus, live latency" codec mode: 2.5 ms frames (120 samples/ch at 48 kHz)
  via the float-input encode path. End-to-end codec delay drops to ~5 ms (vs
  ~12.5 ms at standard 10 ms Opus). Test on LAN: 400 pps/lane, zero missed /
  reordered / duplicate packets, ~15 ms one-way saved end-to-end.

* Wire-format change: AudioFormatInfo.FrameDurationMilliseconds renamed to
  FrameSamplesPerChannel (int sample-count at announced sample rate). Removes
  the lossy 48000*ms/1000 conversion that couldn't represent 2.5 ms. v3 <-> v3
  exact; v3 <-> v2 still passes audio (Opus decoder is self-describing from
  packet TOC) but v2 side over-sizes its buffer wildly. v2.x profiles auto-
  migrate via <120 sentinel rule in RemSoundSettingsStore (anything below 120
  is treated as legacy ms and multiplied by 48). Profile JSON key kept as
  OpusFrameMilliseconds via [JsonPropertyName] so old profile files still load.

* Codec dropdown rebuilt with use-case names: "PCM 48K 24 bit - uncompressed",
  "Opus, broadcast quality - loss tolerant", "Opus, live latency - for jamming
  and monitoring". Middle 10 ms option retired; saved 480-sample profiles
  collapse to broadcast quality (safer-side default).

* Profile auto-resume after self-update: RemSoundUpdater writes a one-shot
  _resume-after-update.txt sentinel containing the active profile title before
  exit; Program.Main reads + deletes it on next start and silently loads that
  profile, skipping the picker. Helper batch's robocopy /XF excludes the
  sentinel and the failure-branch cleans it up if the install aborts. Falls
  through to normal startup behaviour (StartWithProfileTitle or picker) if the
  sentinel is missing, empty, or names a profile that no longer exists.

* Read-only profile saves now go through on explicit Ctrl+S / File -> Save
  with a one-time TaskDialog warning ("Save anyway" / "Cancel" + Do-not-show-
  again). Lock continues to suppress the automatic unsaved-changes prompt on
  close / profile switch (its main job). AppConfig.SaveOnReadOnlyMessageSuppressed
  renamed to SaveOnReadOnlyWarningSuppressed; v2.x suppression flag is silently
  discarded since the behaviour changed and the user needs to see the warning
  once on each machine.

* Manual (readme.html) updated: codec table rewritten with the three new
  choices and corrected bandwidth figures, send-rate description updated, new
  sections "The same profile picks up automatically after an update" and
  "Saving on purpose while a profile is locked".

* Subsumes the never-separately-released v2.2 work: native Opus encoder
  (~97% less per-second memory churn on Opus send path via Concentus.Native),
  efficiency tidy-ups (item 4 ASIO probe rate, item 6 WaitHandle, item 7
  snapshot cache, items 14/16 heartbeat + discovery), legacy cleanup
  (items 30/34/35/36: KeepAlive infrastructure, drift drop/repeat/accumulator
  fields, fan-out cache stat). New diagnostic columns cpu/memMB/wsMB/
  allocKBps/captureMs/sendMs/recvMs/renderMs gated on Enable-logs.

About dialog updated with v3.0 block at top; v2.2 block retained for the
subsumed work.

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

src/RemSound.Core/AudioFormatInfo.cs

@@ -6,6 +6,18 @@ namespace RemSound.Core;
 /// the semantics. The field is wire-backward-compatible: old receivers parse the first 32
 /// bytes of the format payload and ignore the extra; new receivers reading a 32-byte
 /// payload from an old sender default Lane to <see cref="RenderRoute.Mixed"/>.
+///
+/// 2026-05-23 — <c>FrameDurationMilliseconds</c> renamed to <c>FrameSamplesPerChannel</c>
+/// (v3.0 wire-format change). Previously held an int millisecond value; now holds the exact
+/// sample-count per channel at the announced <see cref="SampleRate"/>. Conversion is
+/// <c>ms = FrameSamplesPerChannel * 1000 / SampleRate</c> for display. The change exists
+/// because Opus's 2.5 ms RESTRICTED_LOWDELAY frame (= 120 samples at 48 kHz) can't be
+/// expressed cleanly in integer milliseconds; using sample-count also removes a lossy
+/// conversion that the encoder/decoder pipeline previously did at every announcement.
+/// v2.x receivers reading this field will misinterpret the value as milliseconds and
+/// over-size their internal buffers; the actual decode still works because the Opus
+/// decoder is self-describing from the packet TOC byte. Within-major-version (v3.x ↔ v3.x)
+/// the field is unambiguous.
 /// </summary>
 public sealed record AudioFormatInfo(
     int SampleRate,
@@ -15,9 +27,15 @@ public sealed record AudioFormatInfo(
     int BlockAlign,
     int AverageBytesPerSecond,
     int Codec = (int)AudioTransportCodec.Pcm,
-    int FrameDurationMilliseconds = 10,
+    int FrameSamplesPerChannel = 480,
     RenderRoute Lane = RenderRoute.Mixed)
 {
+    /// <summary>Human-friendly frame duration in milliseconds, derived from
+    /// <see cref="FrameSamplesPerChannel"/> and <see cref="SampleRate"/>. May be a fraction
+    /// (2.5 ms at 48 kHz / 120 samples). For string formatting only — the encoder/decoder
+    /// hot path uses <see cref="FrameSamplesPerChannel"/> directly.</summary>
+    public double FrameDurationMs => SampleRate > 0 ? FrameSamplesPerChannel * 1000.0 / SampleRate : 0;
+
     public override string ToString()
     {
         var encodingName = Encoding switch
@@ -28,7 +46,7 @@ public sealed record AudioFormatInfo(
         };
         var codecName = (AudioTransportCodec)Codec switch
         {
-            AudioTransportCodec.Opus => $" over Opus ({FrameDurationMilliseconds} ms)",
+            AudioTransportCodec.Opus => $" over Opus ({FrameDurationMs:0.##} ms)",
             _ => ""
         };
         var laneName = Lane == RenderRoute.Mixed ? "" : $" [{Lane}]";