feat(multiplayer): reject outdated clients with distinct messaging

Closes §8.43's 'version-mismatch-specific client messaging' gap. Per
the user's explicit go-ahead to design new server behavior for this
(rather than only wiring up something that already existed, the
pattern every other fix this session followed): before this, there
was no server-side protocol rejection at all. queue_create accepted
any protocol_version >= 1 unconditionally, so an outdated client could
only ever discover a mismatch by waiting in the queue forever
unmatched -- the matcher's own compatibility check requires every
formed player to share an identical protocol_version -- with no error
and no explanation given to the player.

Server: Service.MinProtocolVersion (opt-in, zero by default so every
existing caller keeps accepting protocol_version 1 unconditionally)
rejects a below-floor queue_create with 426 Upgrade Required /
client_outdated before the request ever reaches the candidate
provider. Wired via cmd/control-plane's new --min-protocol-version
flag (validated non-negative at startup).

Client: ControlPlaneClient recognises HTTPClient.RESPONSE_UPGRADE_REQUIRED
on queue_create specifically and sets a distinct 'Your client is out
of date -- please update to continue searching' message instead of
the server's raw generic error string, and clears _last_queue_create
so can_retry_queue_create() never offers 'Retry Search' for a failure
that retrying with the same build can never fix.

Verified: go build/vet/test -race clean across every server package.
TestQueueCreateEnforcesMinProtocolVersion covers below-floor rejection
(candidate provider never reached), exactly-at-floor acceptance, and
the error body naming client_outdated; TestQueueCreateMinProtocolVersionZeroIsDisabled
proves the opt-in default doesn't change behavior for every existing
caller. Godot: test_outdated_client_receives_a_distinct_message_and_no_retry_offer
proves the distinct message and suppressed retry offer. Full Godot
suite (217/217, 0 failed, no crash), full make verify-multiplayer-local
gate, zero new crash reports.

Game/tests/cases/test_control_plane_client.gd

@@ -338,6 +338,24 @@ func test_queue_conflict_response_handler_defers_ticket_recovery() -> void:
 	client.free()
 
 
+# Covers §8.43's "version-mismatch-specific client messaging": a 426 Upgrade
+# Required on queue_create (the server-side floor added alongside this test)
+# must surface a distinct, actionable message rather than the server's raw
+# generic error string, and must not offer a futile "Retry Search" -- the
+# same client build will fail again identically every time.
+func test_outdated_client_receives_a_distinct_message_and_no_retry_offer() -> void:
+	var client := ControlPlaneClient.new()
+	client._ready()
+	assert_true(client.configure("https://match.example", "session-id:opaque-token"), "client configures")
+	client._operation = "queue_create"
+	client._last_queue_create = {"ticket_id": "ticket-outdated", "playlist": "casual", "client_build": "build-1", "protocol_version": 4, "key": "outdated-key-123456"}
+	client._on_request_completed(HTTPRequest.RESULT_SUCCESS, HTTPClient.RESPONSE_UPGRADE_REQUIRED, PackedStringArray(), JSON.stringify({"error": "client_outdated"}).to_utf8_buffer())
+	assert_eq(client.state.phase, MatchmakingState.FAILED, "outdated client fails the search")
+	assert_true(client.state.message.to_lower().contains("update"), "message tells the player to update rather than repeating the raw server error: %s" % client.state.message)
+	assert_true(not client.can_retry_queue_create(), "retrying with the same outdated client build is never offered")
+	client.free()
+
+
 func test_rest_responses_reject_malformed_resource_identifiers() -> void:
 	var client := ControlPlaneClient.new()
 	client._ready()