Adopt versioning standard + migrate to versioned protocol planes (0.2.0)

Establishes VERSIONING.md: SemVer 0.x (MINOR = breaking wire change) for the
release version, and per-plane protocol versions enforced on the wire so
incompatible peers fail fast and legibly instead of via silent decode/signature
errors.

⚠️ BREAKING WIRE CHANGE — all peers must run >= 0.2.0 to interoperate (ALPNs and
gossip subscription topics changed). A pre-0.2.0 peer (e.g. an un-resynced
dopedart) can no longer connect, by design, and now fails at the handshake.

- New src/protocol.rs: single source of truth for AUDIO/FRIENDS/GOSSIP_PROTO,
  the derived ALPNs (peerspeak/audio/1, peerspeak/friends/1), GOSSIP_SIG_DOMAIN,
  and versioned_topic(). Unit tests assert ALPN/domain strings match their
  integer versions (no silent drift) + that topic namespacing is deterministic.
- Unified ALPNs: audio was b"peerspeak-audio" (unversioned, and duplicated in
  iroh_impl.rs + core/mod.rs) -> peerspeak/audio/1 from protocol.rs; friends
  re-exports protocol::FRIENDS_ALPN (was peerspeak/friends/0 -> /1).
- Gossip: subscribe to versioned_topic(ticket.topic_id) so different gossip
  versions never share a swarm; the raw topic_id stays the room identity and
  what signatures bind. GOSSIP_SIG_DOMAIN centralized into protocol.rs.
- Cargo.toml 0.1.0 -> 0.2.0.

316 lib tests / clippy --all-targets clean. VERSIONING.md documents the bump
rules, the "I changed X -> what do I bump" table, and a release checklist.

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

src/network/gossip.rs

@@ -12,7 +12,7 @@ use serde::{Serialize, Deserialize};
 
 /// Domain-separation tag mixed into every signed gossip payload so a signature
 /// can never be lifted out of this protocol/version into another context.
-const GOSSIP_SIG_DOMAIN: &str = "peerspeak-gossip-v1";
+use crate::protocol::GOSSIP_SIG_DOMAIN;
 
 /// How far a payload's sender-stamped timestamp may differ from local time
 /// before it's rejected as stale (replayed) or implausibly future. Bounds the
@@ -248,7 +248,11 @@ impl RoomState for IrohGossipState {
             crate::redact_for_log(ticket_str)
         ));
         let ticket = ticket_str.parse::<PeerSpeakTicket>()?;
-        let topic_id = TopicId::from_bytes(ticket.topic_id);
+        // Version-namespace the subscribed topic (VERSIONING.md): peers on a
+        // different gossip protocol version derive a different topic from the same
+        // ticket and never share a swarm. The raw ticket.topic_id stays the room
+        // identity (and what signatures bind, below).
+        let topic_id = TopicId::from_bytes(crate::protocol::versioned_topic(ticket.topic_id));
 
         crate::log_msg(&format!(
             "Parsed ticket. host_id={}, host_addrs={}, topic={}",