feat(multiplayer): report assignment-ready from the supervisor

Closes the second blocker named last commit. Re-traced the actual code
path rather than trusting the earlier assumption: server_boot.gd
verifies its mounted roster file synchronously in _ready(), before
NetworkManager.host() runs and before ServerControl.set_process_ready
is ever called -- so by the time the loopback /ready probe (and thus
Agones Ready, and thus process-ready registration) succeeds, Godot has
already verified its own roster. And the API's ASSIGNMENT_READY gate
(AdvanceServerRegistrationSQL) checks only durable `assignments` rows
server-side, nothing Godot reports. No new Godot-side state was needed
-- the earlier 'needs Godot's own roster-verification state exposed'
claim was overcautious and is corrected here.

The supervisor now calls registerControlPlane(ctx, true) right after
process-ready succeeds, with a bounded retry (default 5 attempts, 2s
apart, both configurable) rather than a single attempt: the durable
`assignments` rows the server-side gate checks may not have propagated
by the first attempt, and that is expected, not fatal. Unlike a
process-ready registration failure, a persistent assignment-ready
failure does NOT kill the child -- the process is already legitimately
listening and usable, and killing a healthy process over a lagging
control-plane read would be actively harmful; it's logged to stderr
instead.

Covered by two tests: the full process-ready-then-assignment-ready
sequence and body shapes, and a retry test that fails the assignment-
ready call twice with 409 (simulating the real gate not yet
satisfied) before succeeding on the third attempt, asserting Start()
still succeeds and the child is never killed.

server/cmd/game-server-supervisor/main.go

@@ -47,6 +47,8 @@ func main() {
 	matchIDEnv := options.String("match-id-env", "COSMIC_CLASH_MATCH_ID", "environment variable containing the allocated match ID; if unset/empty, falls back to the cosmic-clash.io/match-id annotation on the allocated GameServer")
 	protocolVersion := options.Int("protocol-version", 0, "protocol version reported at registration")
 	imageDigestEnv := options.String("image-digest-env", "COSMIC_CLASH_IMAGE_DIGEST", "environment variable containing this build's sha256 image digest")
+	assignmentReadyAttempts := options.Int("assignment-ready-attempts", 5, "retry attempts for assignment-ready registration after process-ready succeeds (a slow-to-propagate signed roster is not fatal)")
+	assignmentReadyBackoff := options.Duration("assignment-ready-backoff", 2*time.Second, "delay between assignment-ready retry attempts")
 	if err := options.Parse(args[:separator]); err != nil {
 		os.Exit(2)
 	}
@@ -70,6 +72,9 @@ func main() {
 		MatchID:           os.Getenv(*matchIDEnv),
 		ProtocolVersion:   *protocolVersion,
 		ImageDigest:       os.Getenv(*imageDigestEnv),
+
+		AssignmentReadyAttempts: *assignmentReadyAttempts,
+		AssignmentReadyBackoff:  *assignmentReadyBackoff,
 	})
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "game-server-supervisor: %v\n", err)