Release v3.3: end-to-end encrypted audio, plus cue and reliability fixes

Headline: all audio is now encrypted (AES-256-GCM), keyed by a per-profile
password. Mandatory — v3.3 only interoperates with v3.3+.

Encryption
- RemSoundCrypto (Core): PBKDF2 key derivation, AES-GCM encrypt/decrypt
  (low-alloc, into-span), password fingerprint, light on-disk obfuscation.
- Wire: SenderLane encrypts the audio payload (PCM split across parts when the
  +28 overhead crosses MTU); AudioReceiver/StreamSession decrypt via a shared
  single-thread AudioDecryptor. Fingerprint piggybacks on the Format packet
  (offset 36, backward-compatible) so a peer can detect a password mismatch.
- Profile.Password (scrambled), carried through BuildCurrentProfile; MainForm
  derives + pushes the key/fingerprint to sender + receiver (RecomputeAudioCrypto).
- UX: ask-for-password on profile create; File -> Change this profile's password
  (ProfilePasswordDialog); Options -> Profile passwords (manager); a gate that
  prompts before streaming without a password; and a clear "passwords don't
  match" / "peer needs to update" message driven by the fingerprint.

Cue fixes
- CuePlayer (NAudio) replaces System.Media.SoundPlayer, which silently failed
  on the 96 kHz/24-bit cue WAVs (and any custom file) — cues now play reliably,
  resampled to 48 kHz/16-bit. Also fixes the Preferences preview button.
- Connect/disconnect cues now audio-gated with hysteresis: connected when audio
  flows OR heartbeat healthy; lost only when audio stops AND heartbeat
  unreachable. Kills false disconnects and the receive-only "no cues" case.
- Honest cue logging (played / muted / not loaded).

Smaller
- Endpoint stickiness: keep the audio target pinned to the heartbeat-proven
  address instead of chasing a multi-homed peer's other (unreachable) address.
- "Online/offline" label now audio+heartbeat aware, not discovery-only.
- "Show what's new after each update" preference (on by default).

Docs: About v3.3 block, RELEASE_NOTES, README (encryption as a headline),
manual section 12 "Passwords and encryption" (+ renumber), MANUAL.md regenerated.
Version 3.2.0 -> 3.3.0.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/RemSound.Receiver/StreamSession.cs

@@ -19,6 +19,7 @@ internal sealed class StreamSession : IDisposable
     private readonly SessionPlayout sessionPlayout;
     private readonly ReceiverDiagnostics diagnostics;
     private readonly Action<int> onFramesQueued;
+    private readonly AudioDecryptor decryptor;
     private readonly PcmFrameAssembler pcmAssembler = new();
     private IOpusDecoder? opusDecoder;
     // Sequence-tracking for Opus FEC recovery. uint, so wrap-around is naturally
@@ -87,7 +88,8 @@ internal sealed class StreamSession : IDisposable
         AudioFormatInfo format,
         SessionPlayout sessionPlayout,
         ReceiverDiagnostics diagnostics,
-        Action<int> onFramesQueued)
+        Action<int> onFramesQueued,
+        AudioDecryptor decryptor)
     {
         Endpoint = endpoint;
         StreamId = streamId;
@@ -95,6 +97,7 @@ internal sealed class StreamSession : IDisposable
         this.sessionPlayout = sessionPlayout;
         this.diagnostics = diagnostics;
         this.onFramesQueued = onFramesQueued;
+        this.decryptor = decryptor;
 
         if (Codec == AudioTransportCodec.Opus)
         {
@@ -211,12 +214,18 @@ internal sealed class StreamSession : IDisposable
             return true; // pending or dropped due to mismatch — not an error condition
         }
 
-        // assembled is signed int24 LE, stereo. Convert to float32 and queue.
-        var sampleCount = assembled.Length / 3;
+        // The reassembled frame is ciphertext — decrypt it. An empty result means the peer's
+        // password doesn't match ours (or we have no key): drop silently. The app surfaces the
+        // mismatch from the fingerprint in the format packet, so it isn't a mystery to the user.
+        var assembledPlain = decryptor.TryDecrypt(assembled);
+        if (assembledPlain.IsEmpty) return false;
+
+        // assembledPlain is signed int24 LE, stereo. Convert to float32 and queue.
+        var sampleCount = assembledPlain.Length / 3;
         var floatBytes = sampleCount * sizeof(float);
         Span<byte> floatScratch = floatBytes <= 16 * 1024 ? stackalloc byte[floatBytes] : new byte[floatBytes];
         var floatSpan = MemoryMarshal.Cast<byte, float>(floatScratch);
-        PcmPack.Int24LEToFloat(assembled, floatSpan);
+        PcmPack.Int24LEToFloat(assembledPlain, floatSpan);
 
         // Discontinuity probe — what does the audio look like right after we decode it?
         // Compared to the sender's pre-encode probe, a higher value here would mean the
@@ -235,6 +244,12 @@ internal sealed class StreamSession : IDisposable
     {
         if (opusDecoder is null) return false;
 
+        // Decrypt the Opus payload up front; both the FEC pass and the normal decode below use
+        // the plaintext. An empty result = wrong password / no key set → drop (silence). The
+        // mismatch is surfaced to the user from the format-packet fingerprint. 2026-05-31.
+        payload = decryptor.TryDecrypt(payload);
+        if (payload.IsEmpty) return false;
+
         // Frame size in samples-per-channel comes directly off the wire in v3.0+ (was
         // SampleRate × ms / 1000 in v2.x). Floor at 120 = 2.5 ms = standard libopus
         // RESTRICTED_LOWDELAY minimum, so a malformed format packet with a tiny value can't