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.Sender/SenderLane.cs

@@ -1,3 +1,4 @@
+using System.Security.Cryptography;
 using RemSound.Core;
 
 namespace RemSound.Sender;
@@ -38,6 +39,16 @@ internal sealed class SenderLane
     private int frameAccumulatorWritten;
     private readonly byte[] outboundScratch = new byte[2048];
 
+    // Audio encryption (always on as of the 2026-05-31 encryption feature). Each lane keeps its
+    // OWN AES-GCM cipher because AES-GCM isn't thread-safe and the two lanes run on separate
+    // capture threads. Rebuilt only when the key reference changes (rare — a password change);
+    // null when no password is set, in which case the lane sends nothing (mandatory encryption).
+    // cipherScratch holds the per-frame ciphertext (plaintext + 28 bytes overhead); 4096 covers
+    // the largest single frame (Opus 20 ms or PCM 5 ms) with room to spare.
+    private AesGcm? cryptoGcm;
+    private byte[]? cryptoKeyCached;
+    private readonly byte[] cipherScratch = new byte[4096];
+
     // Per-stream sequence counters. audioSequence is what the receiver's gap-detector and Opus
     // FEC look at — it must stay monotonic per stream. formatSequence is used for the periodic
     // format-announce packet; receiver doesn't sequence-check format packets but having a
@@ -262,8 +273,21 @@ internal sealed class SenderLane
         {
             PcmPack.FloatToInt24LE(stereoFloats, int24);
         }
+        EnsureCrypto();
+        if (cryptoGcm is null) return; // no password yet → never send audio in the clear
+        // Encrypt the whole PCM frame, then split the ciphertext across as many parts as the
+        // Ethernet payload budget needs (the +28-byte crypto overhead can push a 5 ms frame over
+        // a single datagram). The receiver reassembles the parts and then decrypts.
+        var ctLen = RemSoundCrypto.EncryptInto(cryptoGcm, int24, cipherScratch);
+        var maxPart = RemPacket.MaxAudioPayloadBytes;
+        var totalParts = (byte)((ctLen + maxPart - 1) / maxPart);
         pcmFrameId++;
-        SendPcmPart(pcmFrameId, partIndex: 0, totalParts: 1, int24);
+        for (byte part = 0; part < totalParts; part++)
+        {
+            var offset = part * maxPart;
+            var len = Math.Min(maxPart, ctLen - offset);
+            SendPcmPart(pcmFrameId, part, totalParts, cipherScratch.AsSpan(offset, len));
+        }
     }
 
     private void EmitOpusFrame(ReadOnlySpan<float> stereoFloats)
@@ -282,7 +306,10 @@ internal sealed class SenderLane
             if (len <= 0) return;
             opusBytes = opusEncoder.LastEncoded(len);
         }
-        SendAudio(opusBytes);
+        EnsureCrypto();
+        if (cryptoGcm is null) return; // no password yet → never send audio in the clear
+        var ctLen = RemSoundCrypto.EncryptInto(cryptoGcm, opusBytes, cipherScratch);
+        SendAudio(cipherScratch.AsSpan(0, ctLen));
     }
 
     // === wire path ===
@@ -312,10 +339,34 @@ internal sealed class SenderLane
         // belongs to. The Lane value carried here comes from the AudioFormatInfo constructed
         // above, which currently always sets Mixed for the default lane; Stage 4 will set
         // WasapiLane / AsioLane on the second lane in BothIndependent mode.
-        Span<byte> packet = stackalloc byte[RemPacket.HeaderSize + RemPacket.FormatPayloadExtendedSize];
+        Span<byte> packet = stackalloc byte[RemPacket.HeaderSize + RemPacket.FormatPayloadWithFingerprintSize];
         RemPacket.WriteHeader(packet, RemPacketType.Format, streamId, ++formatSequence);
-        RemPacket.WriteFormatPayload(packet[RemPacket.HeaderSize..], format);
-        owner.SendToAll(packet);
+        // Append our password fingerprint so the peer can tell whether its profile password
+        // matches ours without anyone sending the password. WriteFormatPayload returns 36 (no
+        // fingerprint set) or 44 (fingerprint written); we send exactly that many payload bytes.
+        var payloadLen = RemPacket.WriteFormatPayload(packet[RemPacket.HeaderSize..], format, owner.AudioFingerprint);
+        owner.SendToAll(packet[..(RemPacket.HeaderSize + payloadLen)]);
+    }
+
+    /// <summary>Rebuild this lane's AES-GCM cipher if the owner's audio key reference changed.
+    /// Cheap reference check on the hot path; the actual rebuild only happens on a password
+    /// change. Null key (no password) leaves the cipher null, which stops the lane sending.</summary>
+    private void EnsureCrypto()
+    {
+        var key = owner.AudioKey;
+        if (ReferenceEquals(key, cryptoKeyCached)) return;
+        cryptoGcm?.Dispose();
+        cryptoGcm = key is null ? null : RemSoundCrypto.CreateGcm(key);
+        cryptoKeyCached = key;
+    }
+
+    /// <summary>Release the AES-GCM cipher's native handle. Called from AudioSender.Dispose so
+    /// the handle doesn't leak on teardown (same native-handle discipline as the Opus encoder).</summary>
+    public void DisposeCrypto()
+    {
+        cryptoGcm?.Dispose();
+        cryptoGcm = null;
+        cryptoKeyCached = null;
     }
 
     private void SendPcmPart(uint frameId, byte partIndex, byte totalParts, ReadOnlySpan<byte> partBytes)