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.

src/net/protocol.gd

@@ -3,7 +3,9 @@ extends RefCounted
 ## Wire constants. Bump [constant VERSION] whenever a codec layout changes; the
 ## server refuses mismatched clients at handshake rather than desyncing later.
 
-const VERSION := 1
+## 2: added spawn-grace flag + cleared countdown to the snapshot, and the
+## roster message.
+const VERSION := 2
 const DEFAULT_PORT := 27015
 const MAX_CLIENTS := 32
 
@@ -22,3 +24,12 @@ enum InstanceKind { LOBBY, DUNGEON }
 const F_ALIVE := 1
 const F_INVULN := 2
 const F_ESCAPING := 4
+## Arrival protection: invulnerable and unable to shoot. Distinct from F_INVULN
+## so the client can label the status rather than just tint the ship.
+const F_SPAWN_GRACE := 8
+## The peer behind this player has dropped and is being channelled out.
+const F_LINKDEAD := 16
+
+## Snapshot's cleared-countdown byte when the instance is not in the cleared
+## state -- 30s fits in a byte, so 255 is free to mean "not applicable".
+const COUNTDOWN_NONE := 255