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.

CLAUDE.md

@@ -116,6 +116,9 @@ ticks in milliseconds with no SceneTree.
   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 contact damage.** Every enemy threatens through bullets only; touching
+  one is harmless. `tests/unit/test_content.gd` enforces that every hostile has
+  an emitter.
 - **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.