Fix bullet/ship desync; rework death, escape, arrival and hub awareness

(1) Bullets appeared to trail the ship. Two independent causes, measured with
the new tools/diag_prediction.gd rather than guessed at:
  - ServerRuntime ticked before ClientRuntime, so input sampled on frame N was
    not consumed until frame N+1, leaving the drawn ship a constant one tick
    (4.00px at 240 u/s) ahead of the authoritative one that bullets spawn from.
    ClientRuntime now sets process_physics_priority = -10. Gap on a listen
    server: 4.00px -> 0.10px mean, 0.30px worst.
  - PLAYER_MUZZLE_OFFSET was PLAYER_RADIUS + 6 = 12px against a 13px drawn
    ship, so bullets were born inside the sprite. Regression from the previous
    commit's hitbox shrink; it now derives from PLAYER_VISUAL_RADIUS.

(2) No more timed respawn. A downed player stays down until they ask for the
hub (E), which is an ordinary input -- the server has no "revive me" message.

(3) Escape channel 3s -> 1s, and damage no longer cancels it. An interruptible
channel makes killing the process strictly better than using the button, so a
dropped connection now runs the same channel: the player stays in the world as
linkdead, still killable, and is only released once it completes. Instances
refuse to close while a linkdead body is resolving, or a solo drop would delete
it on the next tick and hand the exploit straight back.

(4) Escape opens an in-game menu: return to hub (routed through the same held-
escape channel, not a new message), disconnect, quit.

(5) Server pushes a roster so the hub shows who is online and which dungeon
they are in. Entering a dungeon grants 2s arrival protection -- invulnerable
AND weapons-cold, since invulnerability alone would make the spawn a free
firing position -- flagged in the snapshot and drawn on every protected ship.

(6) Cleared dungeons hold the party 30s (was 5s) with a visible countdown.

(7) The hub's grey circle was a 100k-HP target dummy that read as scenery. Now
drawn as a bullseye so its purpose is legible.

Protocol version 1 -> 2. 91 tests (was 78); smoke.sh gains a bot that is
SIGKILLed mid-dungeon to prove the disconnect path end to end. check.sh,
test.sh and smoke.sh all pass.

CLAUDE.md

@@ -100,6 +100,21 @@ ticks in milliseconds with no SceneTree.
    strips comments. Anything load-bearing (the 60 Hz tick) is asserted in code
    in `src/main.gd` instead of trusted to `project.godot`.
 
+## Non-obvious invariants
+
+- **`ClientRuntime.process_physics_priority = -10`.** The client must sample and
+  send input before `ServerRuntime` ticks, or a listen server's drawn ship sits
+  a permanent tick ahead of the authoritative one and bullets trail it. Measure
+  with `godot --headless --path . res://tools/diag_prediction.tscn` (~0.1px is
+  healthy, 4px means the ordering broke).
+- **`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.
+- **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.
+  See [docs/NETCODE.md](docs/NETCODE.md).
+
 ## Adding content
 
 A new enemy or boss is data, never code. Add a builder to