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/core/game_opts.gd

@@ -17,6 +17,11 @@ static var bot_client: bool = false
 static var autoquit_ticks: int = 0
 ## Skip the menu and connect straight away. Implied by --bot.
 static var autojoin: bool = false
+## Bot harness only: gracefully disconnect after this many ticks inside a
+## dungeon, the way the in-game menu's "Disconnect" button does. Lets the smoke
+## test cover the clean-exit path as well as a hard kill -- both must be caught
+## by the same server-side escape channel.
+static var bot_leave_after: int = 0
 ## Skip the menu and start a listen server (host + local player).
 static var listen: bool = false
 ## Server-side dev switch: new dungeons open straight onto the boss, skipping
@@ -38,6 +43,10 @@ static func parse(argv: PackedStringArray = PackedStringArray()) -> void:
 				autojoin = true
 			"--join":
 				autojoin = true
+			"--leave-after":
+				i += 1
+				if i < argv.size():
+					bot_leave_after = int(argv[i])
 			"--listen":
 				listen = true
 			"--boss-rush":