feat(join-auth): add key-ID rotation to signed join authorisations

Prerequisite for wiring the allocator to publish rosters. The signing
key is a shared HMAC secret mounted into both the allocator and the
allocated game server; without a key ID, rotating it would invalidate
every authorisation already issued for an in-flight match, because a
server holding only the new key cannot verify a token signed with the
old one.

Add KeyID to JoinAuthorisation and append it to the canonical claim
bytes, so it is covered by the signature and cannot be repointed at a
different key than the one that actually signed. Allocated servers now
hold a set of currently-valid keys and select by ID: a rotation
publishes the new key alongside the old, and the old is dropped once no
live match can still reference it.

The key file becomes a JSON map of key ID to base64 key. A file of raw
key bytes is still accepted as a single key under the empty ID, which is
what an unrotated deployment and the kind fixture use.

Game/scripts/match_net.gd builds the canonical bytes independently, so
it changes in lockstep; the cross-language golden token in
test_match_net.gd is regenerated from the Go implementation and now
carries a key ID. Added tests cover accepting either key mid-rotation,
rejecting a retired key ID, and rejecting a token whose key ID was
swapped to name a key the server does hold.

Go suite and 223 Godot tests pass.

server/domain/reconnect.go

@@ -35,6 +35,12 @@ type JoinAuthorisation struct {
 	Protocol   string
 	Generation uint64
 	ExpiresAt  time.Time
+	// KeyID names the signing key so the allocator can rotate without
+	// invalidating authorisations already issued for in-flight matches: the
+	// game server holds a set of currently-valid keys and selects by this ID.
+	// It is part of the signed bytes, so it cannot be swapped to point at a
+	// different key than the one that actually signed.
+	KeyID string
 }
 
 type rankedConnection struct {