feat(client): add rendezvous traversal coordinators (#12)

docs/architecture/0008-scoped-join-attempts-and-tickets.md

@@ -24,18 +24,24 @@ credentials plus opaque attempt and mediation IDs from a process-ephemeral HMAC
 key, the client subject, the complete canonical request fingerprint, a fresh salt,
 and a purpose/role label. Credentials are 32-byte base64url values (43 characters),
 below both the 192-character Rendezvous capability ceiling and LiteNetLib's
-256-character NAT token ceiling. State retains keyed credential fingerprints,
-derivation inputs, and salt—not issued plaintext. All diagnostic string
-representations redact credentials and derivation material.
+256-character NAT token ceiling. The connection ticket uses half of that payload
+for its attempt ID and half for an independently derived 128-bit authenticator, so
+the SDK can correlate concurrent introductions without increasing UDP response
+size. State retains keyed credential fingerprints, derivation inputs, and salt—not
+issued plaintext. All diagnostic string representations redact credentials and
+derivation material.
 
 The client receives only its punch capability. A host polls its own listing with
 the lease token in `X-Rendezvous-Lease-Token` and receives only host-role
 capabilities through a signed, listing-bound, five-minute cursor. Replaying an
 identical join request returns the same live attempt; changing the request under
 the same owner/key conflicts. A client may cancel with its punch capability in
-`X-Rendezvous-Client-Punch-Capability`; cancellation atomically removes the
-attempt. Listing deletion, expiry, revocation, or process restart removes every
-associated attempt and credential fingerprint.
+`X-Rendezvous-Client-Punch-Capability`; cancellation atomically marks the attempt
+and retains a bounded tombstone until its original expiry. Host polling returns
+that tombstone so a coordinator can revoke any local ticket authorization, while
+endpoint binding, introduction, ticket issuance, and ticket consumption all
+reject the cancelled attempt. Listing deletion, expiry, revocation, or process
+restart removes every associated attempt and credential fingerprint.
 
 Endpoint binding remains role- and capability-specific. The first endpoint
 observed for a role wins atomically; an exact UDP duplicate is idempotent, while
@@ -50,8 +56,14 @@ fingerprint-consumption seam for mediator tests and revocation. On the game host
 the SDK's bounded `ConnectionTicketValidator` stores a process-keyed digest,
 accepts an exact ticket once under a lock, rejects altered/cross-attempt/expired/
 revoked/replayed tickets, and zeroes retained digests and key material on disposal.
-Issue #11 carries the ticket in the authenticated introduction; issue #12 wires
-authorization and consumption into the caller-owned LiteNetLib coordinator.
+Issue #11 carries the fixed-size ticket in the authenticated introduction. Issue
+#12 extracts its embedded attempt ID, bounds the host's local authorization window
+by both the host-polled attempt expiry and the configured ticket lifetime, then
+wires one-time consumption into the caller-owned coordinator. Both peers receive
+a digest of the exact expected ticket over HTTP and reject any syntactically valid
+but unauthenticated introduction token. Embedding the ID prevents concurrent or
+late introductions from cross-binding a valid ticket while preserving the
+mediator's 2.0 response-byte amplification ceiling.
 
 ## Consequences