Bring wire protocol into full YAW/2 spec compliance

Five interop issues fixed against PROTOCOL.md:

- Join signature now covers nonce_raw || net_ascii (64-char UTF-8 hex
  string) as specified in §5.1, not net_raw_bytes. Both anchor server
  and client updated to match.

- Chat wire fields renamed to spec names: text/ts (Unix ms int64)
  replacing body/sent_at (ISO timestamp). Flat layout on PeerMessage
  matches §8 exactly; store and TUI updated accordingly.

- Direct messages now use the spec "pm" type (flat {type,mid,text,ts})
  instead of chat+to. Receiver reconstructs a ChatMessage with
  dm:<short-id> room for IPC/storage. §8 compliant.

- File transfer message types changed to spec hyphenated names:
  file-offer, file-accept, file-cancel, file-done with spec field
  names (name/size not filename/size_bytes). §9 compliant.

- DataChannel open-race (§14 gotcha #3) fixed with sync.Once: doOpen
  fires on OnOpen callback or immediately if the channel is already
  open when WireDataChannel is called (answerer race).

Also fixes two bugs found during testing:

- mid was missing from outgoing wire messages, causing all received
  messages to arrive with mid="" and collide on the UNIQUE DB
  constraint. mid is now included on all sent chat/pm messages; a
  random mid is generated for any received message that omits it.

- Test scripts hardened: kill -9 + active lsof polling replaces blind
  sleep for port cleanup; join_network sent before peer_field queries
  (local_peer is now network-scoped and nil until joined).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/anchor/main.go

@@ -170,9 +170,8 @@ func (a *anchor) handleWS(w http.ResponseWriter, r *http.Request) {
 				log.Printf("anchor: join: bad sig from %s", r.RemoteAddr)
 				continue
 			}
-			// Sig covers nonce || net (both as raw bytes decoded from hex/plaintext).
-			netBytes, _ := hex.DecodeString(msg.Net)
-			signed := append(nonce, netBytes...)
+			// §5.1: sig covers nonce_raw || net_ascii (net as 64-char hex UTF-8 string)
+			signed := append(nonce, []byte(msg.Net)...)
 			if !ed25519.Verify(ed25519.PublicKey(pubBytes), signed, sigBytes) {
 				log.Printf("anchor: join: sig verification failed for %s", msg.ID[:min(8, len(msg.ID))])
 				continue