fix(agones): stop a dead health loop from passing as a healthy server

Every allocated GameServer reached Ready and was recycled by Agones ~20s
later. Health pings are the game process's job by design -- the
supervisor has no health implementation at all -- so a server that stops
pinging is exactly what Agones is built to reclaim.

start_health() armed a Timer on a node that might not be inside the
SceneTree. A Timer only ticks inside the tree, so the node reported
itself configured, sent nothing, and said nothing about it. It now
returns a bool, refuses loudly when unconfigured, and defers to _ready()
when called before parenting, so the SDK arms its own timer and no
caller has to get the ordering right. server_boot.gd defers the add like
every sibling does (§9 gotcha 27) and logs when AGONES_SDK_HTTP_PORT is
missing, which previously read identically to a healthy start.

Also bounded the in-flight latch: it is set across an await, so a request
that never completes would silence health permanently. Defence in depth
rather than an observed fault.

Tests target the contract rather than the mechanism: a test that parents
the SDK correctly and asserts pings passes with the bug present, because
the defect was in the wiring. The unit tests assert start_health()
cannot claim success out of tree, and were confirmed to fail against the
previous code. The smoke gains a counting sidecar and asserts a
*repeating* ping -- it reports "health pings in 3.0s = 1, want at least
2" when the loop is broken, which is the production symptom exactly. It
is also now actually run: nothing referenced it before.

Two diagnostic fixes, both of which changed conclusions during this work:

The kind gate only built the game-server image when the tag was absent,
so a local rerun silently verified whatever was built last. That is why
local runs and CI disagreed about the same commit. It now builds by
default, with KIND_REUSE_GAME_SERVER_IMAGE=1 as the opt-in fast path.

The failure dump logged only not-ready pods, and used --all-containers
with a shared tail. A GameServer recycled after reaching Ready leaves no
unready pod behind, and the Agones sidecar out-logs the game server, so
the relevant output was never captured. It now dumps every pod, per
container, current and previous, plus the GameServer and Fleet resources
-- Agones' own state machine is what rejects these.

scripts/verify_multiplayer_local.sh

@@ -48,6 +48,18 @@ echo "local multiplayer gate: bounded fuzz targets"
 echo "local multiplayer gate: Godot harness"
 run_godot_harness
 
+# The Agones SDK smoke needs a live SceneTree and awaits an HTTP round trip, so
+# it cannot live in test_runner.tscn -- that runner calls test methods without
+# awaiting. It covers the property the unit tests structurally cannot: that
+# start_health() produces a *repeating* ping, which is what Agones enforces and
+# whose absence silently recycled every allocated GameServer.
+echo "local multiplayer gate: Agones SDK smoke"
+if [[ -x "$godot_bin" ]]; then
+  "$godot_bin" --headless --path "$root_dir/Game" --script res://tests/agones_sdk_smoke.gd
+else
+  echo "local multiplayer gate: skipping Agones SDK smoke, Godot executable not found ($godot_bin)" >&2
+fi
+
 echo "local multiplayer gate: contracts and manifests"
 python3 -m json.tool "$root_dir/server/contracts/v1/openapi.json" >/dev/null
 # json.tool only proves the contract parses. test_contracts.py is what actually