feat: v0.7.2 — portmapper (UPnP+PCP+NAT-PMP), session-relay opt-in, URL Phase 1

Network/reachability improvements + a relay-privacy fix. Wire-compatible
with v0.7.0/v0.7.1; no protocol changes.

- Replace hand-rolled UPnP (igd-next) with the portmapper crate. All three
  protocols (UPnP-IGD / NAT-PMP / PCP) run in parallel, auto-renew
  internally. PCP adds IPv6 firewall pinholes and works on iOS without
  the multicast entitlement. Pulled in transitively via iroh already, no
  net dep growth.
- Android: UPnP/PCP/NAT-PMP attempted on WiFi/Ethernet with a
  WifiManager.MulticastLock acquired for the lifetime of the mapping.
  Cellular skipped early (no UPnP/PCP gateway, avoid 3s discovery waste).
- TCP port-mapping gate removed for mobile — phones with permissive NAT
  can now serve HTTP for direct browser fetches.
- Anchor reachability watcher (bidirectional): clears is_anchor after
  >5min of no port mapping; restores it when the mapping comes back.
  Network roams self-heal without restart. Mobile never auto-anchors.
- Session relay opt-in restored. relay.session_relay_enabled setting
  defaults OFF (anchors included — servers shouldn't silently burn
  bandwidth either). Gates both serving (can_accept_relay_pipe) and
  using (auto-fallback in node.rs). UI toggle in Settings. Relay-style
  signaling (RelayIntroduce / worm_lookup / N1-N3 shares) unaffected.
- URL Phase 1: share links now contain only the post ID
  (itsgoin.net/p/<post>). Anchor handler already supported post-ID-only
  URLs (author was optional); just dropped the author hex from the
  generator. Older URLs with author hex continue to work.
- Quick app close button in header (with confirm) — useful for stopping
  network activity between sessions on mobile.
- JNI null-pointer guards on ndk_context handles in android_wifi.rs.

MEMORY rule sharpened to distinguish session relay (byte pipe, opt-in)
from relay-style signaling/discovery (always on).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/core/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "itsgoin-core"
-version = "0.7.1"
+version = "0.7.2"
 edition = "2021"
 
 [dependencies]
@@ -19,7 +19,11 @@ ed25519-dalek = { version = "=3.0.0-pre.1", features = ["rand_core", "zeroize"]
 chacha20poly1305 = "0.10"
 base64 = "0.22"
 zip = { version = "2", default-features = false, features = ["deflate"] }
-igd-next = { version = "0.16", features = ["tokio"] }
+portmapper = "0.14"
+
+[target.'cfg(target_os = "android")'.dependencies]
+jni = "0.21"
+ndk-context = "0.1"
 
 [dev-dependencies]
 tempfile = "3"