feat(trust): host-gated trust-on-first-use — PIN pairing mandatory by default

TOFU let anyone who could reach the host click "Trust" and stream, which defeats the point
on a LAN. Make SPAKE2 PIN pairing the default and only way to trust a NEW host; TOFU survives
as an explicit HOST opt-in (for fully trusted networks), advertised over mDNS so clients render
their trust UI from the host's policy rather than offering trust on faith.

Contract:
- Host advertises pair=required (default) or pair=optional. pair=required rejects unpaired
  clients at the handshake; pair=optional accepts them (TOFU).
- Clients: a pinned host whose fingerprint matches connects silently; a pinned host whose
  fingerprint CHANGED forces re-pairing via PIN (no re-trust shortcut); a NEW host is offered
  TOFU only if it advertised pair=optional, otherwise PIN pairing is mandatory; a manually-typed
  or unknown-policy host is always PIN.

Host (crates/punktfunk-host/src/main.rs):
- m3-host now REQUIRES pairing by default (was open by default). New --allow-tofu opts into
  accepting unpaired clients + advertising pair=optional; pairing is always armed (PIN logged at
  startup). serve --native was already secure-by-default (serve --open). The mDNS advert and the
  accept loop already mapped require_pairing -> pair=required + reject; only the m3-host CLI
  default + help text changed.

Clients honor the advertised policy:
- Android (MainActivity.kt): TOFU only for a discovered pair=optional host; manual/unknown -> PIN;
  fp-change -> re-pair only (dropped the "Forget & re-TOFU" shortcut).
- Apple (HostDiscovery/SessionModel/ContentView/HostCards/HostStore): new allowsTofu
  (pair==optional, distinct from unknown); connect() gates .awaitingTrust on it; unpinned
  non-optional hosts route to the PIN sheet; "Forget Identity" re-pairs rather than re-TOFUs.
- Linux (app.rs/ui_hosts.rs/session.rs): ConnectRequest.pair_required -> pair_optional;
  initiate_connect routes pinned/fp-changed/optional/else; manual + --connect unknown -> PIN; a
  pinned connect rejected on trust grounds re-pairs.

Docs (CLAUDE.md, README.md, docs-site/content/docs/pairing.md): describe the gated model — PIN is
the default, TOFU an explicit opt-in with an impostor warning.

Verified: host cargo check/clippy/fmt clean; Android built + live (emulator -> home-worker-2):
a manual connect now opens the PIN dialog (no Trust button) and the PIN ceremony streams; Apple
swift build clean; Linux clippy -D warnings + fmt clean on the Linux box.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-client-linux/src/session.rs

@@ -42,7 +42,13 @@ pub enum SessionEvent {
         mode: Mode,
         fingerprint: [u8; 32],
     },
-    Failed(String),
+    /// `trust_rejected` is set when the connect failed the TLS trust check (a `Crypto`
+    /// error): for a pinned connect this is the fingerprint-changed signal, so the UI can
+    /// offer a re-pair (PIN) path rather than a dead-end error.
+    Failed {
+        msg: String,
+        trust_rejected: bool,
+    },
     Ended(Option<String>),
     Stats(Stats),
 }
@@ -97,6 +103,7 @@ fn pump(
     ) {
         Ok(c) => Arc::new(c),
         Err(e) => {
+            let trust_rejected = matches!(e, PunktfunkError::Crypto);
             let msg = match e {
                 PunktfunkError::Crypto => {
                     "Host identity rejected — wrong fingerprint, or the host requires pairing"
@@ -105,7 +112,10 @@ fn pump(
                 PunktfunkError::Timeout => "Connection timed out".to_string(),
                 other => format!("Connect failed: {other:?}"),
             };
-            let _ = ev_tx.send_blocking(SessionEvent::Failed(msg));
+            let _ = ev_tx.send_blocking(SessionEvent::Failed {
+                msg,
+                trust_rejected,
+            });
             return;
         }
     };