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/net/protocol.gd

@@ -4,8 +4,10 @@ extends RefCounted
 ## server refuses mismatched clients at handshake rather than desyncing later.
 
 ## 2: added spawn-grace flag + cleared countdown to the snapshot, and the
-## roster message.
-const VERSION := 2
+##    roster message.
+## 3: dropped the post-hit invulnerability flag, added the respawn-lockout byte
+##    to each player record.
+const VERSION := 3
 const DEFAULT_PORT := 27015
 const MAX_CLIENTS := 32
 
@@ -22,10 +24,9 @@ enum InstanceKind { LOBBY, DUNGEON }
 
 ## Player flags packed into the snapshot's per-player byte.
 const F_ALIVE := 1
-const F_INVULN := 2
 const F_ESCAPING := 4
-## Arrival protection: invulnerable and unable to shoot. Distinct from F_INVULN
-## so the client can label the status rather than just tint the ship.
+## Arrival protection: invulnerable and unable to shoot. The only invulnerable
+## state there is -- see the i-frames note in SimConfig.
 const F_SPAWN_GRACE := 8
 ## The peer behind this player has dropped and is being channelled out.
 const F_LINKDEAD := 16