chore(warnings): sweep the workspace — zero warnings on lib + bin targets

Addressed every rustc warning surfaced by \`cargo check --workspace
--release --lib --bins\` on opus-DRED-v2. Split across three
categories:

## Real bugs surfaced by the audit (fix, don't silence)

- **crates/wzp-relay/src/federation.rs** — the per-peer RTT monitor
  task computed \`rtt_ms\` every 5 s and threw it on the floor. The
  \`wzp_federation_peer_rtt_ms\` gauge has been registered in
  metrics.rs the whole time but was never receiving samples, leaving
  the Grafana panel blank. Wired it up: the task now calls
  \`fm_rtt.metrics.federation_peer_rtt_ms.with_label_values(&[&label_rtt]).set(rtt_ms)\`
  on every sample. Fixes three warnings (\`rtt_ms\`, \`fm_rtt\`,
  \`label_rtt\` were all captured for this task and all dead).

## Dead code removal

- **crates/wzp-relay/src/federation.rs** — removed \`local_delivery_seq:
  AtomicU16\` field and its initializer. It was described in comments
  as "per-room seq counter for federation media delivered to local
  clients" but was declared, initialized to 0, and never read or
  written anywhere else. Genuine half-wired feature; deletable with
  zero behavior change.
- **crates/wzp-relay/src/room.rs** — removed \`let recv_start =
  Instant::now()\` at the top of a recv loop that was never read.
  Separate variable \`last_recv_instant\` already measures the actual
  gap that's used for the \`max_recv_gap_ms\` stat.
- **crates/wzp-client/src/cli.rs** — removed \`let my_fp = fp.clone()\`
  from the signal loop setup. Cloned but never used in any match arm.

## Stub-intent warnings (underscore + explanatory comment)

- **crates/wzp-relay/src/handshake.rs** — \`choose_profile\` hardcodes
  \`QualityProfile::GOOD\` and ignores its \`supported\` parameter.
  Comment already documented "Cap at GOOD (24k) for now — studio
  tiers not yet tested for federation reliability". Renamed to
  \`_supported\`, expanded the comment to explicitly note the future
  plan (pick highest supported ≤ relay ceiling).
- **crates/wzp-relay/src/federation.rs** — \`forward_to_peers\` takes
  \`room_name: &str\` but only uses \`room_hash\`. The caller
  (handle_datagram) passes the name for caller-site symmetry with
  other helpers; kept the param shape and underscored the binding
  with a comment noting it's reserved for future per-name logging.

## Cosmetic fixes

- **crates/wzp-relay/src/event_log.rs** — dropped \`use std::sync::Arc\`
  (unused).
- **crates/wzp-relay/src/signal_hub.rs** — trimmed \`use tracing::{info,
  warn}\` to \`use tracing::info\`. Also removed unnecessary \`mut\` on
  \`hub\` binding in the \`register_unregister\` test.
- **crates/wzp-relay/src/room.rs** — trimmed \`use tracing::{debug,
  error, info, trace, warn}\` to \`{error, info, warn}\`. Also removed
  unnecessary \`mut\` on \`mgr\` binding in the \`room_join_leave\` test.
- **crates/wzp-relay/src/main.rs** — removed unnecessary \`mut\` on the
  \`config\` destructured binding from \`parse_args()\`; and dropped
  \`ref caller_alias\` from the \`DirectCallOffer\` match pattern since
  the relay just forwards the full \`msg\` (caller_alias is preserved
  end-to-end, we don't need to read it on the relay).
- **crates/wzp-crypto/tests/featherchat_compat.rs** — dropped
  \`CallSignalType\` from a \`use wzp_client::featherchat::{...}\`
  (unused in the test body). Note: this test file has pre-existing
  compile errors from SignalMessage schema drift unrelated to this
  sweep; that's tracked separately.

## Crate-level annotation

- **crates/wzp-android/src/lib.rs** — added
  \`#![allow(dead_code, unused_imports, unused_variables, unused_mut)]\`
  with a doc block explaining the crate is dead code since the Tauri
  mobile rewrite. The legacy Kotlin+JNI Android app that consumed
  this crate was replaced by desktop/src-tauri (live Android recv
  path) + crates/wzp-native (Oboe bridge). Rather than piecemeal
  cleanup of a crate that shouldn't be maintained, the whole-crate
  allow keeps CI clean until someone removes the crate entirely. Kills
  all 6 wzp-android warnings (4 unused imports/vars, 1 unused \`mut\`
  on a JNI env param, 1 dead \`command_rx\` field) in one line.

## Not touched

- **deps/featherchat/warzone/crates/warzone-protocol/src/x3dh.rs** —
  3 unused-variable warnings in \`alice_spk_secret\`, \`alice_bundle\`,
  \`bob_bundle_bytes\`. This is a vendored third-party submodule;
  upstream's problem, not ours. Would need to be reported to
  featherchat upstream if we care.

## Verification

- \`cargo check --workspace --release --lib --bins\` → 0 warnings, 0 errors
- \`cargo check --workspace --release --all-targets\` → only the 3
  featherchat submodule warnings remain, plus the pre-existing 3
  broken integration tests (SignalMessage schema drift from Phase 2,
  tracked separately and explicitly out of scope).

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

crates/wzp-relay/src/federation.rs

@@ -142,9 +142,6 @@ pub struct FederationManager {
     peer_links: Arc<Mutex<HashMap<String, PeerLink>>>,
     /// Dedup filter for incoming federation datagrams.
     dedup: Mutex<Deduplicator>,
-    /// Per-room seq counter for federation media delivered to local clients.
-    /// Ensures clients see monotonically increasing seq regardless of federation sender.
-    local_delivery_seq: std::sync::atomic::AtomicU16,
     /// JSONL event log for protocol analysis.
     event_log: EventLogger,
     /// Per-room rate limiters for inbound federation media.
@@ -172,7 +169,6 @@ impl FederationManager {
             metrics,
             peer_links: Arc::new(Mutex::new(HashMap::new())),
             dedup: Mutex::new(Deduplicator::new(DEDUP_WINDOW_SIZE)),
-            local_delivery_seq: std::sync::atomic::AtomicU16::new(0),
             event_log,
             rate_limiters: Mutex::new(HashMap::new()),
         }
@@ -296,7 +292,12 @@ impl FederationManager {
     /// Forward locally-generated media to all connected peers.
     /// For locally-originated media, we send to ALL peers (they decide whether to deliver).
     /// For forwarded media (multi-hop), handle_datagram filters by active_rooms.
-    pub async fn forward_to_peers(&self, room_name: &str, room_hash: &[u8; 8], media_data: &Bytes) {
+    ///
+    /// `_room_name` is kept in the signature for caller-site symmetry with
+    /// the other room-tagged helpers and for future per-room-name logging
+    /// or rate limiting; the body currently forwards on `room_hash` alone
+    /// because that's what the wire format carries.
+    pub async fn forward_to_peers(&self, _room_name: &str, room_hash: &[u8; 8], media_data: &Bytes) {
         let links = self.peer_links.lock().await;
         if links.is_empty() {
             return;
@@ -623,11 +624,20 @@ async fn run_federation_link(
         }
     };
 
-    // RTT monitor: periodically sample QUIC RTT for this peer
+    // RTT monitor: periodically sample QUIC RTT for this peer and push it
+    // into the `wzp_federation_peer_rtt_ms` gauge. The gauge is registered
+    // in metrics.rs but previously never received any samples — the task
+    // computed rtt_ms and dropped it on the floor, leaving the Grafana
+    // panel blank. Fixed as part of the workspace warning sweep.
     let rtt_task = async move {
         loop {
             tokio::time::sleep(Duration::from_secs(5)).await;
             let rtt_ms = rtt_transport.connection().stats().path.rtt.as_millis() as f64;
+            fm_rtt
+                .metrics
+                .federation_peer_rtt_ms
+                .with_label_values(&[&label_rtt])
+                .set(rtt_ms);
         }
     };