Fix permanent input-timing desync; no i-frames; UI respawn; guard dead joins

The real cause of the ship/bullet separation, which the previous commit only
half-addressed. The server dropped inputs past a lead of 12 while the client
only re-synced past 16, so a client whose lead drifted into 13-16 had every
input silently rejected while believing its timing was fine. The server coasted
on held_input and then stopped; the client kept predicting. The two separated
permanently and the reconciler fought it every snapshot -- "shoved around".
It needed two independent clocks to drift, hence "only after some time", and
nothing in the loop could notice, hence "then persists". The listen-server
diagnostic could never reproduce it: one process, one physics tick, lead
constant by construction.

Two defences: INPUT_MAX_LEAD (40) is now far wider than the client's correction
band (3..20), asserted by tests/unit/test_input_lead.gd so narrowing it fails a
test; and an ack-stall detector re-syncs when last_input_tick stops advancing,
which catches the whole class regardless of cause -- lead alone cannot, because
a wrong lead looks normal from the client. diag_prediction.gd now injects a +14
tick drift and exits non-zero unless the gap recovers.

Also:
- No invulnerability frames. Every bullet that touches a player lands; i-frames
  made dense patterns safer than sparse ones, which inverts the genre. Measured:
  a stationary player survives ~13.6s of the Warden's opening phase, ~17.5s
  drifting. spawn_grace remains the only invulnerable state.
- Death is exited with a HUD button, disabled for the first 3s. The lockout is
  enforced in SimWorld, not just by graying the button -- a client that ignores
  its own UI still waits. The interact key no longer respawns.
- Joining a server that is not there no longer drops the player into an empty
  lobby they cannot act in. Net.join() only creates an ENet object; the game
  scene now waits for the server to actually place us in an instance, with an
  8s timeout, and headless runs exit non-zero instead of idling.

Protocol 2 -> 3. 98 tests; check.sh, test.sh and smoke.sh all pass.

src/sim/sim_world.gd

@@ -149,8 +149,6 @@ func step() -> void:
 
 func _step_players() -> void:
 	for p in players.values():
-		if p.iframes > 0:
-			p.iframes -= 1
 		if p.spawn_grace > 0:
 			p.spawn_grace -= 1
 		if p.fire_cooldown > 0:
@@ -159,10 +157,13 @@ func _step_players() -> void:
 		var frame := _take_input(p)
 
 		if not p.alive:
+			if p.respawn_lockout > 0:
+				p.respawn_lockout -= 1
 			# No timed respawn: a downed player waits for the hub. Asking to go
 			# is an input like any other, so a dead client cannot be revived by
-			# anything except its own request reaching the server.
-			if frame.pressed(InputFrame.BTN_INTERACT) or p.linkdead:
+			# anything except its own request reaching the server -- and not
+			# before the lockout expires, however early its UI lets it ask.
+			if p.can_request_respawn() and (frame.pressed(InputFrame.BTN_INTERACT) or p.linkdead):
 				events.append({"t": SimEvent.Type.RESPAWN_REQUESTED, "peer": p.peer_id})
 			continue
 
@@ -372,11 +373,11 @@ func _kill_bullet(slot: int) -> void:
 ## SimConfig.ESCAPE_CHANNEL_TICKS for why that would reward pulling the plug.
 func _damage_player(p: SimPlayer, amount: int) -> void:
 	p.hp = maxi(p.hp - amount, 0)
-	p.iframes = SimConfig.PLAYER_IFRAMES
 	events.append({"t": SimEvent.Type.PLAYER_HIT, "peer": p.peer_id, "dmg": amount, "hp": p.hp})
 	if p.hp <= 0:
 		p.alive = false
 		p.escape_ticks = 0
+		p.respawn_lockout = SimConfig.RESPAWN_LOCKOUT_TICKS
 		events.append({"t": SimEvent.Type.PLAYER_DIED, "peer": p.peer_id})