fix(security): authenticate gossip payloads (S2 — sign + verify, replay-bound)

The gossip `author` field was self-asserted, so an in-room member could forge
it to impersonate (chat), force-evict (Leave), or poison presence + the address
book (Announce). Now every GossipPayload is signed with the node's ed25519
secret key and verified on receipt; a forged author can't validate because the
attacker lacks the victim's key.

Done as the complete fix (forgery + replay):
- GossipPayload gains `ts` (sender-stamped) + `sig` (iroh::Signature, serde-64B);
  Debug is hand-written since Signature has none.
- Signature covers domain tag + room topic_id + author + ts + msg
  (`signable_bytes`): topic binding blocks cross-room replay, ts + a 2-min
  freshness window block temporal replay (within-window replays are byte-
  identical and de-duped by the swarm), author binding makes spoofing fail.
- New pure seams `sign_gossip` / `verify_gossip` (+ `GossipReject`); all four
  outgoing broadcasts sign, the receive loop verifies-then-trusts (drops
  unauthenticated/stale before any peer-map / event / address-book action).
- IrohGossipState now holds the node SecretKey + active topic bytes; callers
  (core, test_net) updated.

NOTE: breaking gossip wire change — all peers must run this build (the staged
friend release + dopedart need rebuild). Node identity is ephemeral, so no
migration concern beyond rebuild.

+5 unit tests (genuine accept; forged author, tampered msg, cross-room, stale/
future all rejected). 163 lib tests (was 158), clippy --all-targets clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/bin/test_net.rs

@@ -14,7 +14,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let lookup_a = iroh::address_lookup::memory::MemoryLookup::new();
     let secret_a = iroh::SecretKey::generate();
     let endpoint_a = Endpoint::builder(presets::N0)
-        .secret_key(secret_a)
+        .secret_key(secret_a.clone())
         .address_lookup(lookup_a.clone())
         .bind()
         .await?;
@@ -27,13 +27,13 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         .accept(iroh_gossip::net::GOSSIP_ALPN, gossip_a.clone())
         .spawn();
 
-    let room_a = IrohGossipState::new(endpoint_a.clone(), gossip_a.clone(), lookup_a.clone());
+    let room_a = IrohGossipState::new(endpoint_a.clone(), gossip_a.clone(), lookup_a.clone(), secret_a);
 
     // 2. Node B (Client) Setup
     let lookup_b = iroh::address_lookup::memory::MemoryLookup::new();
     let secret_b = iroh::SecretKey::generate();
     let endpoint_b = Endpoint::builder(presets::N0)
-        .secret_key(secret_b)
+        .secret_key(secret_b.clone())
         .address_lookup(lookup_b.clone())
         .bind()
         .await?;
@@ -46,7 +46,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         .accept(iroh_gossip::net::GOSSIP_ALPN, gossip_b.clone())
         .spawn();
 
-    let room_b = IrohGossipState::new(endpoint_b.clone(), gossip_b.clone(), lookup_b.clone());
+    let room_b = IrohGossipState::new(endpoint_b.clone(), gossip_b.clone(), lookup_b.clone(), secret_b);
 
     // 3. Create room on Node A
     let topic_id = rand::random();