Unlock menu after failed join; remove contact damage; cover clean disconnects

A failed connection left the connect buttons disabled forever. _show_menu()
returned early when a menu already existed, so the set_busy(false) that
re-enables them never ran on the way back from _abort_connect -- the player was
locked out of both joining and hosting with no way out but a restart. Also
guards against a second attempt stacking on an in-flight one.

Contact damage is gone as a concept: nothing in this game hurts you by touching
it, because every threat should be a bullet you can see and dodge. The Stalker
walked at you and dealt contact damage and nothing else, so it now carries a
point-blank shotgun instead -- five pellets, 62 degrees, 18-tick lifetime for
about 78px of reach, so it still has to close and leaves nothing lingering.
tests/unit/test_content.gd asserts every hostile enemy has an emitter, so a new
enemy cannot quietly reintroduce the mechanic.

The menu's "Disconnect" DOES get the anti-cheese protection -- verified, not
assumed. It calls Net.shutdown(), the socket closes, and the server takes the
same linkdead path as a SIGKILL, because the channel is keyed on the socket
closing rather than on how it closed. Made permanent: a --leave-after bot flag
and two smoke assertions covering the polite exit alongside the hard kill, so a
future "clean leave" message that skipped the channel would fail a test. The
menu now says so out loud in a dungeon -- the mechanic only reads as fair if
the cost is known before clicking.

103 tests, 12 smoke assertions; check.sh, test.sh and smoke.sh all pass.

src/sim/sim_world.gd

@@ -140,7 +140,6 @@ func step() -> void:
 		_step_boss()
 		pool.step()
 		_resolve_bullet_hits()
-		_resolve_contact_damage()
 		_emit_spawn_events()
 	else:
 		# Replica: bullets only. Actor state arrives in snapshots.
@@ -352,17 +351,6 @@ func _resolve_bullet_hits() -> void:
 				_kill_bullet(i)
 
 
-func _resolve_contact_damage() -> void:
-	for e in enemies.values():
-		if not e.alive or e.def.contact_damage <= 0:
-			continue
-		for p in players.values():
-			if not p.alive or p.invulnerable():
-				continue
-			if Movement.circles_overlap(e.pos, e.def.radius, p.pos, SimConfig.PLAYER_RADIUS):
-				_damage_player(p, e.def.contact_damage)
-
-
 ## Bullets removed early must be announced -- clients cannot derive a hit.
 func _kill_bullet(slot: int) -> void:
 	events.append({"t": SimEvent.Type.BULLET_DESPAWN, "uid": pool.uid[slot]})