feat(net): Phase 7 — dual-socket IPv4+IPv6 ICE

Adds a dedicated IPv6 QUIC endpoint (IPV6_V6ONLY=1 via socket2)
alongside the existing IPv4 signal endpoint for proper dual-stack
P2P connectivity. Previous [::]:0 dual-stack attempt broke IPv4
on Android; this uses separate sockets per address family like
WebRTC/libwebrtc.

- create_ipv6_endpoint(): socket2-based IPv6-only UDP socket,
  tries same port as IPv4 signal EP, falls back to ephemeral
- local_host_candidates(v4_port, v6_port): now gathers IPv6
  global-unicast (2000::/3) and unique-local (fc00::/7) addrs
- dual_path::race(): A-role accepts on both v4+v6 via select!,
  D-role routes each candidate to matching-AF endpoint
- Graceful fallback: if IPv6 unavailable, .ok() → None → pure
  IPv4 behavior identical to pre-Phase-7

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

Cargo.lock

@@ -2566,7 +2566,7 @@ dependencies = [
  "libc",
  "percent-encoding",
  "pin-project-lite",
- "socket2",
+ "socket2 0.6.3",
  "system-configuration",
  "tokio",
  "tower-service",
@@ -4240,7 +4240,7 @@ dependencies = [
  "quinn-udp",
  "rustc-hash",
  "rustls",
- "socket2",
+ "socket2 0.6.3",
  "thiserror 2.0.18",
  "tokio",
  "tracing",
@@ -4279,7 +4279,7 @@ dependencies = [
  "cfg_aliases",
  "libc",
  "once_cell",
- "socket2",
+ "socket2 0.6.3",
  "tracing",
  "windows-sys 0.60.2",
 ]
@@ -5241,6 +5241,16 @@ version = "1.15.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "67b1b7a3b5fe4f1376887184045fcf45c69e92af734b7aaddc05fb777b6fbd03"
 
+[[package]]
+name = "socket2"
+version = "0.5.10"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e22376abed350d73dd1cd119b57ffccad95b4e585a7cda43e286245ce23c0678"
+dependencies = [
+ "libc",
+ "windows-sys 0.52.0",
+]
+
 [[package]]
 name = "socket2"
 version = "0.6.3"
@@ -6000,7 +6010,7 @@ dependencies = [
  "parking_lot",
  "pin-project-lite",
  "signal-hook-registry",
- "socket2",
+ "socket2 0.6.3",
  "tokio-macros",
  "windows-sys 0.61.2",
 ]
@@ -7810,6 +7820,7 @@ dependencies = [
  "rustls",
  "serde_json",
  "sha2",
+ "socket2 0.5.10",
  "tokio",
  "tracing",
  "wzp-proto",