feat: implement scoped join attempts and tickets (#10)

Closes #10

docs/contracts/http-v1.md

@@ -33,6 +33,7 @@ the same value as a required query parameter.
 | `GET` | `/v1/sessions` | Browse compatible public sessions. |
 | `GET` | `/v1/sessions/{listingId}` | Resolve a public or explicitly shared unlisted listing. |
 | `POST` | `/v1/join-attempts` | Authorize and create a short-lived join attempt. |
+| `DELETE` | `/v1/join-attempts/{attemptId}` | Cancel an attempt using its client punch capability. |
 | `GET` | `/v1/sessions/{listingId}/join-attempts` | Let an authenticated host poll pending attempts. |
 | `POST` | `/v1/join-attempts/{attemptId}/outcome` | Report a bounded connection outcome. |
 | `GET` | `/health/live` | Report that the HTTP process is alive. |
@@ -49,6 +50,11 @@ for mutation operations are carried in their request bodies. Public browser
 responses contain no IP endpoints, lease tokens, punch capabilities, connection
 tickets, player identifiers, or gameplay state.
 
+Attempt cancellation sends the short-lived client punch capability in
+`X-Rendezvous-Client-Punch-Capability`. Join creation uses the observed HTTP
+source only for a process-keyed, short-lived idempotency/abuse scope; this is not
+player authentication and is never returned to callers.
+
 ## Idempotency, cursors, and retries
 
 Registration and join creation require a caller-generated visible-ASCII