refactor: extract shared engine helpers, federation clone-before-send, constants

Engine deduplication (PRD-engine-dedup.md):
- build_call_config(): shared CallConfig construction (was 23 lines × 2)
- codec_to_profile(): shared CodecId → QualityProfile mapping (was 19 lines × 2)
- run_signal_task(): shared signal handler (was 48 lines × 2)
- Net -39 lines from engine.rs, 6 duplicated blocks → single-line calls

Quick wins from REFACTOR-codebase-audit.md:
- 6 magic number constants extracted (CAPTURE_POLL_MS, RECV_TIMEOUT_MS, etc.)
- DRED_POLL_INTERVAL moved from 2 local defs to 1 module-level const
- federation.rs: forward_to_peers, broadcast_signal, send_signal_to_peer
  now clone peer list and release lock before sending (was holding Mutex
  across async I/O — last lock-during-send pattern eliminated)
- main.rs: close_transport() helper replaces 12 silent .ok() calls with
  debug-level logging

314 tests passing, 0 regressions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/PRD-engine-dedup.md

@@ -0,0 +1,132 @@
+# PRD: Engine.rs Deduplication — Extract Shared Send/Recv Helpers
+
+## Problem
+
+`desktop/src-tauri/src/engine.rs` is 1,705 lines with two nearly identical `CallEngine::start()` implementations — one for Android (880 lines) and one for desktop (430 lines). ~350 lines are copy-pasted between them. Every change to the encode/decode/adaptive-quality pipeline requires editing both places, and they've already diverged in subtle ways (Android has extensive first-join diagnostics that desktop lacks).
+
+## Scope
+
+Extract the duplicated logic into shared helper functions. The Android and desktop paths should only differ in their audio I/O mechanism (Oboe ring via wzp-native vs CPAL capture_ring/playout_ring).
+
+## What's Duplicated
+
+| Block | Description | Lines (each) |
+|-------|-------------|------|
+| `build_call_config()` | Resolve quality string → CallConfig | 23 |
+| Codec-to-profile match | Map CodecId → QualityProfile for decoder switch | 19 |
+| Adaptive quality switch | Read AtomicU8, index_to_profile, set_profile, update frame_samples + dred_tuner | 15 |
+| DRED tuner poll | Check frame counter, poll quinn stats, apply tuning | 15 |
+| Quality report ingestion | Extract quality_report, feed to AdaptiveQualityController, store to AtomicU8 | 8 |
+| Signal task | Accept signals, handle RoomUpdate/QualityDirective/Hangup | 48 |
+| **Total** | | **~128 lines × 2 = 256 lines eliminated** |
+
+## Implementation
+
+### Phase 1: Top-Level Helper Functions
+
+```rust
+fn build_call_config(quality: &str) -> CallConfig {
+    let profile = resolve_quality(quality);
+    match profile {
+        Some(p) => CallConfig {
+            noise_suppression: false,
+            suppression_enabled: false,
+            ..CallConfig::from_profile(p)
+        },
+        None => CallConfig {
+            noise_suppression: false,
+            suppression_enabled: false,
+            ..CallConfig::default()
+        },
+    }
+}
+
+fn codec_to_profile(codec: CodecId) -> QualityProfile {
+    match codec {
+        CodecId::Opus24k => QualityProfile::GOOD,
+        CodecId::Opus6k => QualityProfile::DEGRADED,
+        CodecId::Opus32k => QualityProfile::STUDIO_32K,
+        CodecId::Opus48k => QualityProfile::STUDIO_48K,
+        CodecId::Opus64k => QualityProfile::STUDIO_64K,
+        CodecId::Codec2_1200 => QualityProfile::CATASTROPHIC,
+        CodecId::Codec2_3200 => QualityProfile {
+            codec: CodecId::Codec2_3200,
+            fec_ratio: 0.5,
+            frame_duration_ms: 20,
+            frames_per_block: 5,
+        },
+        other => QualityProfile { codec: other, ..QualityProfile::GOOD },
+    }
+}
+
+fn check_adaptive_switch(
+    pending: &AtomicU8,
+    encoder: &mut CallEncoder,
+    tuner: &mut wzp_proto::DredTuner,
+    frame_samples: &mut usize,
+    tx_codec: &tokio::sync::Mutex<String>,
+) -> bool {
+    let p = pending.swap(PROFILE_NO_CHANGE, Ordering::Acquire);
+    if p == PROFILE_NO_CHANGE { return false; }
+    if let Some(new_profile) = index_to_profile(p) {
+        let new_fs = (new_profile.frame_duration_ms as usize) * 48;
+        if encoder.set_profile(new_profile).is_ok() {
+            *frame_samples = new_fs;
+            tuner.set_codec(new_profile.codec);
+            // Caller updates tx_codec display string
+            return true;
+        }
+    }
+    false
+}
+```
+
+### Phase 2: Shared Signal Task
+
+Extract the signal task into a standalone async function:
+
+```rust
+async fn run_signal_task(
+    transport: Arc<wzp_transport::QuinnTransport>,
+    running: Arc<AtomicBool>,
+    pending_profile: Arc<AtomicU8>,
+    participants: Arc<Mutex<Vec<ParticipantInfo>>>,
+) {
+    loop {
+        if !running.load(Ordering::Relaxed) { break; }
+        match tokio::time::timeout(
+            Duration::from_millis(SIGNAL_TIMEOUT_MS),
+            transport.recv_signal(),
+        ).await {
+            Ok(Ok(Some(msg))) => {
+                // Handle RoomUpdate, QualityDirective, Hangup...
+            }
+            _ => {}
+        }
+    }
+}
+```
+
+### Phase 3: Shared DRED Poll + Quality Ingestion
+
+These are small blocks but appear in both send and recv tasks. Extract as inline helpers or closures.
+
+## Verification
+
+1. `cargo check --workspace` — must compile
+2. `cargo test -p wzp-proto -p wzp-relay -p wzp-client --lib` — must pass
+3. Manual test: place a call Android↔Desktop, verify audio works in both directions
+4. Verify adaptive quality still switches (set one side to auto, degrade network)
+
+## Effort
+
+- Phase 1: 1 hour (extract 3 functions, update 6 call sites)
+- Phase 2: 30 min (extract signal task, update 2 spawn sites)
+- Phase 3: 30 min (cleanup remaining small duplicates)
+- Total: ~2 hours
+
+## Not In Scope
+
+- Audio I/O trait abstraction (Oboe vs CPAL) — different project, different risk profile
+- Moving Android-specific diagnostics (first-join, PCM recorder) into a feature flag
+- Splitting engine.rs into multiple files