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/sim/sim_world.gd

@@ -334,6 +334,16 @@ func _step_boss() -> void:
 	var phase := boss.current_phase()
 	if phase == null:
 		return
+	# A boss never leaves its arena. Enforced here rather than left to each
+	# boss's movement code, because boss rooms deliberately do not lock: the
+	# player can always walk out, and the fight only stays a fight if the boss
+	# cannot follow. Currently a no-op (every boss is stationary), which is
+	# exactly when an invariant is cheapest to establish.
+	if boss.room.size != Vector2.ZERO:
+		boss.pos = boss.room.position + Vector2(
+			clampf(boss.pos.x - boss.room.position.x, 0.0, boss.room.size.x),
+			clampf(boss.pos.y - boss.room.position.y, 0.0, boss.room.size.y))
+
 	if boss.phase_tick >= phase.telegraph_ticks:
 		var local := posmod(boss.phase_tick - phase.telegraph_ticks, maxi(phase.loop_ticks, 1))
 		_run_emitters(phase.emitters, boss.pos, local, boss.room)