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/presence_net.rs

@@ -31,7 +31,7 @@ use std::time::Duration;
 
 /// ALPN for the friends presence/control plane. Separate from the audio/gossip
 /// ALPNs so a control dial never lands on a bare room endpoint and vice versa.
-pub const FRIENDS_ALPN: &[u8] = b"peerspeak/friends/0";
+pub const FRIENDS_ALPN: &[u8] = crate::protocol::FRIENDS_ALPN;
 
 /// Upper bound on a single control message — generous for a Pong carrying a
 /// member ticket (~300 chars), but rejects a peer trying to make us buffer a