Close Stage 1: per-peer actor interest, boss room confinement

The gap Stage 1 left open: snapshots were encoded once and broadcast to every
peer in an instance, so clients were handed enemies the fog then hid. Fog is a
rendering rule -- a modified client draws whatever it holds -- so that was no
defence at all. Snapshots are now encoded per peer, and actors beyond
ACTOR_INTEREST_RADIUS (800u) are never sent. Measured cost of four filtered
encodes against one shared: 95.6us vs 24.9us, or 0.032 ms/tick amortised over
the snapshot interval.

Two asymmetries worth keeping:

- The observer's own player record is never filtered, however far out the
  arithmetic puts it. The client reconciles its prediction against that record,
  so dropping it breaks the player's own movement rather than hiding someone.
- Only bullet SPAWNS are filtered, never despawns. A client told about a bullet
  must always be told it died, or it keeps a phantom.

Bullet spawns use a much wider radius (2200u) than actors, deliberately. An
enemy appearing at the edge of sight is cosmetic; a bullet withheld at spawn
that later flies into view is invisible damage. The floor is longest bullet
travel + fog radius -- 1500 for the Warden's Collapse snipe -- and
test_interest.gd recomputes that from live content, so adding a faster or
longer-lived bullet fails a test instead of producing bullets that wink into
existence.

Also clamped bosses to their room. A no-op today since every boss is
stationary, which is exactly when the invariant is cheap to establish: boss
rooms have no door that locks, so the only thing keeping a fight in the boss
room is the boss, and Stage 5's movement work would otherwise break it quietly.

146 tests (was 137). check.sh, test.sh and smoke.sh pass. Stage 1 has no
partials left; docs/ROADMAP.md now records the three interest radii and the
floor each must respect.

src/core/sim_config.gd

@@ -117,6 +117,26 @@ const DUNGEON_CLEARED_EXIT_TICKS := 1800  # 30 seconds
 ## always holds the terrain it is about to reveal.
 const FOG_VIEW_RADIUS := 460.0
 
+# --- Interest management ----------------------------------------------------
+## Actors further than this from a player are not sent to that player at all.
+## Comfortably beyond FOG_VIEW_RADIUS so nothing visibly pops in at the edge of
+## sight, but far short of a dungeon, so a modified client cannot read enemy
+## positions across the map.
+const ACTOR_INTEREST_RADIUS := 800.0
+
+## Bullet spawns are filtered far more generously than actors, and deliberately
+## so. A bullet is announced once, at spawn, and then simulated locally -- if we
+## withhold it and it later flies into view, it is invisible damage, which is a
+## gameplay bug rather than a cosmetic one. An enemy appearing at the edge of
+## sight is neither.
+##
+## The floor is (longest bullet travel + FOG_VIEW_RADIUS): a bullet spawned
+## further away than that cannot reach visible ground before it expires.
+## test_interest.gd computes that from the real content and asserts this covers
+## it, so adding a faster or longer-lived bullet fails a test instead of
+## producing bullets that wink into existence.
+const BULLET_INTEREST_RADIUS := 2200.0
+
 # --- Map streaming ----------------------------------------------------------
 ## How far around a player the server streams map tiles. Comfortably wider than
 ## the viewport so movement prediction and bullet simulation always run against