feat(p2p): Phase 5.5 — ICE LAN host candidates (IPv4 + IPv6)

Same-LAN P2P was failing because MikroTik masquerade (like most
consumer NATs) doesn't support NAT hairpinning — the advertised
WAN reflex addr is unreachable from a peer on the same LAN as
the advertiser. Phase 5 got us Cone NAT classification and fixed
the measurement artifact, but same-LAN direct dials still had
nowhere to land.

Phase 5.5 adds ICE-style host candidates: each client enumerates
its LAN-local network interface addresses, includes them in the
DirectCallOffer/Answer alongside the reflex addr, and the
dual-path race fans out to ALL peer candidates in parallel.
Same-LAN peers find each other via their RFC1918 IPv4 + ULA /
global-unicast IPv6 addresses without touching the NAT at all.

Dual-stack IPv6 is in scope from the start — on modern ISPs
(including Starlink) the v6 path often works even when v4
hairpinning doesn't, because there's no NAT on the v6 side.

## Changes

### `wzp_client::reflect::local_host_candidates(port)` (new)

Enumerates network interfaces via `if-addrs` and returns
SocketAddrs paired with the caller's port. Filters:

- IPv4: RFC1918 (10/8, 172.16/12, 192.168/16) + CGNAT (100.64/10)
- IPv6: global unicast (2000::/3) + ULA (fc00::/7)
- Skipped: loopback, link-local (169.254, fe80::), public v4
  (already covered by reflex-addr), unspecified

Safe from any thread, one `getifaddrs(3)` syscall.

### Wire protocol (wzp-proto/packet.rs)

Three new `#[serde(default, skip_serializing_if = "Vec::is_empty")]`
fields, backward-compat with pre-5.5 clients/relays by
construction:

- `DirectCallOffer.caller_local_addrs: Vec<String>`
- `DirectCallAnswer.callee_local_addrs: Vec<String>`
- `CallSetup.peer_local_addrs: Vec<String>`

### Call registry (wzp-relay/call_registry.rs)

`DirectCall` gains `caller_local_addrs` + `callee_local_addrs`
Vec<String> fields. New `set_caller_local_addrs` /
`set_callee_local_addrs` setters. Follow the same pattern as
the reflex addr fields.

### Relay cross-wiring (wzp-relay/main.rs)

Both the local-call and cross-relay-federation paths now track
the local_addrs through the registry and inject them into the
CallSetup's peer_local_addrs. Cross-wiring is identical to the
existing peer_direct_addr logic — each party's CallSetup
carries the OTHER party's LAN candidates.

### Client side (desktop/src-tauri/lib.rs)

- `place_call`: gathers local host candidates via
  `local_host_candidates(signal_endpoint.local_addr().port())`
  and includes them in `DirectCallOffer.caller_local_addrs`.
  The port match is critical — it's the Phase 5 shared signal
  socket, so incoming dials to these addrs land on the same
  endpoint that's already listening.
- `answer_call`: same, AcceptTrusted only (privacy mode keeps
  LAN addrs hidden too, for consistency with the reflex addr).
- `connect` Tauri command: new `peer_local_addrs: Vec<String>`
  arg. Builds a `PeerCandidates` bundle and passes it to the
  dual-path race.
- Recv loop's CallSetup handler: destructures + forwards the
  new field to JS via the signal-event payload.

### `dual_path::race` (wzp-client/dual_path.rs)

Signature change: takes `PeerCandidates` (reflex + local Vec)
instead of a single SocketAddr. The D-role branch now fans out
N parallel dials via `tokio::task::JoinSet` — one per candidate
— and the first successful dial wins (losers are aborted
immediately via `set.abort_all()`). Only when ALL candidates
have failed do we return Err; individual candidate failures are
just traced at debug level and the race waits for the others.

LAN host candidates are tried BEFORE the reflex addr in
`PeerCandidates::dial_order()` — they're faster when they work,
and the reflex addr is the fallback for the not-on-same-LAN
case.

### JS side (desktop/main.ts)

`connect` invoke now passes `peerLocalAddrs: data.peer_local_addrs ?? []`
alongside the existing `peerDirectAddr`.

### Tests

All existing test callsites updated for the new Vec<String>
fields (defaults to Vec::new() in tests — they don't exercise
the multi-candidate path). `dual_path.rs` integration tests
wrap the single `dead_peer` / `acceptor_listen_addr` in a
`PeerCandidates { reflexive: Some(_), local: Vec::new() }`.

Full workspace test: 423 passing (same as before 5.5).

## Expected behavior on the reporter's setup

Two phones behind MikroTik, both on the same LAN:

  place_call:host_candidates {"local_addrs": ["192.168.88.21:XXX", "2001:...:YY:XXX"]}
  recv:DirectCallAnswer {"callee_local_addrs": ["192.168.88.22:ZZZ", "2001:...:WW:ZZZ"]}
  recv:CallSetup {"peer_direct_addr":"150.228.49.65:NN",
                  "peer_local_addrs":["192.168.88.22:ZZZ","2001:...:WW:ZZZ"]}
  connect:dual_path_race_start {"peer_reflex":"...","peer_local":[...]}
  dual_path: direct dial succeeded on candidate 0   ← LAN v4 wins
  connect:dual_path_race_won {"path":"Direct"}

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

crates/wzp-client/tests/dual_path.rs

@@ -19,7 +19,7 @@
 use std::net::{Ipv4Addr, SocketAddr};
 use std::time::Duration;
 
-use wzp_client::dual_path::{race, WinningPath};
+use wzp_client::dual_path::{race, PeerCandidates, WinningPath};
 use wzp_client::reflect::Role;
 use wzp_transport::{create_endpoint, server_config};
 
@@ -110,7 +110,10 @@ async fn dual_path_direct_wins_on_loopback() {
     // should win.
     let result = race(
         Role::Dialer,
-        acceptor_listen_addr,
+        PeerCandidates {
+            reflexive: Some(acceptor_listen_addr),
+            local: Vec::new(),
+        },
         relay_addr,
         "test-room".into(),
         "call-test".into(),
@@ -148,7 +151,10 @@ async fn dual_path_relay_wins_when_direct_is_dead() {
 
     let result = race(
         Role::Dialer,
-        dead_peer,
+        PeerCandidates {
+            reflexive: Some(dead_peer),
+            local: Vec::new(),
+        },
         relay_addr,
         "test-room".into(),
         "call-test".into(),
@@ -182,7 +188,10 @@ async fn dual_path_errors_cleanly_when_both_paths_dead() {
     let start = std::time::Instant::now();
     let result = race(
         Role::Dialer,
-        dead_peer,
+        PeerCandidates {
+            reflexive: Some(dead_peer),
+            local: Vec::new(),
+        },
         dead_relay,
         "test-room".into(),
         "call-test".into(),