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.

CLAUDE.md

@@ -110,6 +110,15 @@ ticks in milliseconds with no SceneTree.
 - **`PLAYER_RADIUS` (hitbox) < `PLAYER_VISUAL_RADIUS` (sprite), and
   `PLAYER_MUZZLE_OFFSET` derives from the visual one.** Prefer a visible
   near-miss over an invisible hit; keep the muzzle clear of the sprite.
+- **`INPUT_MAX_LEAD` must stay well above `INPUT_LEAD_MAX`.** The server's
+  input acceptance window has to be wider than the band in which the client
+  re-syncs its own numbering. Violate it and drifting clocks land in a silent
+  dead zone where the server rejects everything and the client never notices —
+  the ship and the authoritative position separate permanently. Pinned by
+  `tests/unit/test_input_lead.gd`.
+- **No i-frames.** Every bullet that touches a player lands; `spawn_grace` is
+  the only invulnerable state. Do not reintroduce post-hit immunity — it makes
+  dense patterns safer than sparse ones.
 - **A disconnect is not an exit.** Dropping in a dungeon keeps the player in the
   world as `linkdead`, channelling out over the same second the escape costs.
   Damage must never cancel the escape channel, or quitting beats the button.