Fix bullet/ship desync; rework death, escape, arrival and hub awareness

(1) Bullets appeared to trail the ship. Two independent causes, measured with
the new tools/diag_prediction.gd rather than guessed at:
  - ServerRuntime ticked before ClientRuntime, so input sampled on frame N was
    not consumed until frame N+1, leaving the drawn ship a constant one tick
    (4.00px at 240 u/s) ahead of the authoritative one that bullets spawn from.
    ClientRuntime now sets process_physics_priority = -10. Gap on a listen
    server: 4.00px -> 0.10px mean, 0.30px worst.
  - PLAYER_MUZZLE_OFFSET was PLAYER_RADIUS + 6 = 12px against a 13px drawn
    ship, so bullets were born inside the sprite. Regression from the previous
    commit's hitbox shrink; it now derives from PLAYER_VISUAL_RADIUS.

(2) No more timed respawn. A downed player stays down until they ask for the
hub (E), which is an ordinary input -- the server has no "revive me" message.

(3) Escape channel 3s -> 1s, and damage no longer cancels it. An interruptible
channel makes killing the process strictly better than using the button, so a
dropped connection now runs the same channel: the player stays in the world as
linkdead, still killable, and is only released once it completes. Instances
refuse to close while a linkdead body is resolving, or a solo drop would delete
it on the next tick and hand the exploit straight back.

(4) Escape opens an in-game menu: return to hub (routed through the same held-
escape channel, not a new message), disconnect, quit.

(5) Server pushes a roster so the hub shows who is online and which dungeon
they are in. Entering a dungeon grants 2s arrival protection -- invulnerable
AND weapons-cold, since invulnerability alone would make the spawn a free
firing position -- flagged in the snapshot and drawn on every protected ship.

(6) Cleared dungeons hold the party 30s (was 5s) with a visible countdown.

(7) The hub's grey circle was a 100k-HP target dummy that read as scenery. Now
drawn as a bullseye so its purpose is legible.

Protocol version 1 -> 2. 91 tests (was 78); smoke.sh gains a bot that is
SIGKILLed mid-dungeon to prove the disconnect path end to end. check.sh,
test.sh and smoke.sh all pass.

tests/unit/test_server_authority.gd

@@ -122,17 +122,70 @@ func test_a_replica_world_never_resolves_a_hit() -> void:
 		"only the server decides damage; a client replica must never apply it")
 
 
-func test_player_death_and_respawn() -> void:
+func _kill_player() -> SimPlayer:
 	var p: SimPlayer = world.players[PEER]
 	p.pos = Vector2.ZERO
 	p.hp = 5
 	p.iframes = 0
+	p.spawn_grace = 0
 	world.pool.spawn(Vector2.ZERO, Vector2.ZERO, 6.0, 60, 25,
 		SimConfig.TEAM_ENEMY, SimConfig.KIND_ORB)
 	world.step()
-	assert_false(p.alive)
-	for _i in SimConfig.PLAYER_RESPAWN_DELAY + 1:
+	assert_false(p.alive, "setup: the player should be down")
+	return p
+
+
+func _events_of(type: int) -> Array:
+	return world.events.filter(func(e: Dictionary) -> bool: return int(e["t"]) == type)
+
+
+func test_a_downed_player_stays_down_without_input() -> void:
+	var p := _kill_player()
+	world.drain_events()
+	for _i in 600:
 		world.step()
-	assert_true(p.alive)
-	assert_eq(p.hp, SimConfig.PLAYER_MAX_HP)
-	assert_eq(p.pos, world.spawn_point)
+	assert_false(p.alive, "there is no timed respawn -- death waits for the player")
+	assert_eq(_events_of(SimEvent.Type.RESPAWN_REQUESTED).size(), 0)
+
+
+func test_a_downed_player_asking_to_respawn_is_reported_once_per_tick() -> void:
+	_kill_player()
+	world.drain_events()
+	_drive(1, Vector2.ZERO, InputFrame.BTN_INTERACT)
+	assert_eq(_events_of(SimEvent.Type.RESPAWN_REQUESTED).size(), 1,
+		"the request is an event for the instance layer, not a local revive")
+	# Crucially the world does NOT revive the player itself: only the server's
+	# instance layer can, by moving them to the hub.
+	assert_false(world.players[PEER].alive)
+
+
+func test_respawn_request_never_reaches_the_client() -> void:
+	_kill_player()
+	_drive(1, Vector2.ZERO, InputFrame.BTN_INTERACT)
+	var packet := NetCodec.decode_events(NetCodec.encode_events(world.tick, world.events))
+	for ev: Dictionary in packet["events"]:
+		assert_ne(int(ev["t"]), SimEvent.Type.RESPAWN_REQUESTED,
+			"where a dead player goes is the server's decision")
+
+
+func test_spawn_grace_blocks_damage_and_firing() -> void:
+	var p: SimPlayer = world.players[PEER]
+	p.pos = Vector2.ZERO
+	p.iframes = 0
+	p.spawn_grace = 60
+	world.pool.spawn(Vector2.ZERO, Vector2.ZERO, 6.0, 60, 25,
+		SimConfig.TEAM_ENEMY, SimConfig.KIND_ORB)
+	_drive(1, Vector2.ZERO, InputFrame.BTN_FIRE)
+	assert_eq(p.hp, SimConfig.PLAYER_MAX_HP, "arrival protection must absorb the hit")
+	assert_eq(world.pool.live_count, 1,
+		"only the enemy bullet: a protected player cannot shoot either")
+
+
+func test_spawn_grace_expires() -> void:
+	var p: SimPlayer = world.players[PEER]
+	p.spawn_grace = 5
+	_drive(6)
+	assert_eq(p.spawn_grace, 0)
+	assert_false(p.invulnerable())
+	_drive(1, Vector2.ZERO, InputFrame.BTN_FIRE)
+	assert_eq(world.pool.live_count, 1, "the gun comes back once grace ends")