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/content/content.gd

@@ -41,7 +41,6 @@ static func drifter() -> EnemyDef:
 	d.display_name = "Drifter"
 	d.max_hp = 40
 	d.radius = 14.0
-	d.contact_damage = 8
 	d.move = EnemyDef.Move.DRIFT
 	d.speed = 55.0
 	d.visual = 0
@@ -68,7 +67,6 @@ static func turret() -> EnemyDef:
 	d.display_name = "Turret"
 	d.max_hp = 70
 	d.radius = 16.0
-	d.contact_damage = 0
 	d.move = EnemyDef.Move.STATIC
 	d.speed = 0.0
 	d.visual = 1
@@ -88,18 +86,40 @@ static func turret() -> EnemyDef:
 	return d
 
 
-## Walks at you and does nothing else. The pressure enemy.
+## Walks at you and fires a point-blank burst. The pressure enemy.
+##
+## It used to deal contact damage and nothing else. Nothing in this game hurts
+## you by touching you -- every threat is a bullet you can see coming and dodge,
+## which is the whole contract of the genre. So the Stalker's melee is a
+## shotgun with a deliberately tiny lifetime: ~78px of travel, which only
+## threatens once it has closed the distance, and leaves nothing lingering in
+## the arena afterwards.
 static func stalker() -> EnemyDef:
 	var d := EnemyDef.new()
 	d.id = ENEMY_STALKER
 	d.display_name = "Stalker"
 	d.max_hp = 30
 	d.radius = 12.0
-	d.contact_damage = 16
 	d.move = EnemyDef.Move.APPROACH
 	d.speed = 95.0
 	d.retarget_interval = 20
 	d.visual = 2
+	d.pattern_loop_ticks = 45
+
+	var lunge := AimedSpreadEmitter.new()
+	lunge.start_tick = 0
+	lunge.interval = 45
+	lunge.count = 5
+	lunge.spread_deg = 62.0
+	lunge.speed = 260.0
+	lunge.radius = 6.0
+	lunge.damage = 14
+	# 18 ticks at 260 u/s is about 78px of reach -- shorter than the muzzle-to-
+	# player distance at any range you would call "not point blank".
+	lunge.lifetime = 18
+	lunge.muzzle_offset = 10.0
+	lunge.kind = SimConfig.KIND_HEAVY
+	d.emitters = [lunge]
 	return d
 
 
@@ -110,7 +130,6 @@ static func dummy() -> EnemyDef:
 	d.display_name = "Target Dummy"
 	d.max_hp = 100000
 	d.radius = 20.0
-	d.contact_damage = 0
 	d.move = EnemyDef.Move.STATIC
 	d.visual = 3
 	return d