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/net/net_codec.gd

@@ -10,15 +10,36 @@ extends RefCounted
 
 ## [param cleared_countdown] is whole seconds until a cleared dungeon returns
 ## its party, or Protocol.COUNTDOWN_NONE when that does not apply.
+## [param for_peer] scopes the snapshot to what that player may know: actors
+## beyond SimConfig.ACTOR_INTEREST_RADIUS are omitted entirely, so fog is not
+## the only thing hiding them. Pass 0 to encode the whole world (tests, and the
+## safe fallback if the observer cannot be found).
 static func encode_snapshot(world: SimWorld,
-		cleared_countdown: int = Protocol.COUNTDOWN_NONE) -> PackedByteArray:
+		cleared_countdown: int = Protocol.COUNTDOWN_NONE,
+		for_peer: int = 0) -> PackedByteArray:
 	var b := StreamPeerBuffer.new()
 	b.big_endian = false
 	b.put_u32(world.tick)
 	b.put_u8(clampi(cleared_countdown, 0, Protocol.COUNTDOWN_NONE))
 
-	b.put_u8(mini(world.players.size(), 255))
+	var observer: SimPlayer = world.players.get(for_peer) if for_peer != 0 else null
+	var eye := Vector2.ZERO
+	var cull_sq := 0.0
+	if observer != null:
+		eye = observer.pos
+		cull_sq = SimConfig.ACTOR_INTEREST_RADIUS * SimConfig.ACTOR_INTEREST_RADIUS
+
+	var visible_players: Array[SimPlayer] = []
 	for p in world.players.values():
+		# Always include the observer's own player, whatever the distance
+		# arithmetic says: the client reconciles its prediction against this
+		# record, and dropping it would break its own movement.
+		if observer == null or p.peer_id == for_peer \
+				or eye.distance_squared_to(p.pos) <= cull_sq:
+			visible_players.append(p)
+
+	b.put_u8(mini(visible_players.size(), 255))
+	for p in visible_players:
 		b.put_u32(p.peer_id)
 		b.put_float(p.pos.x)
 		b.put_float(p.pos.y)
@@ -43,8 +64,11 @@ static func encode_snapshot(world: SimWorld,
 
 	var live_enemies: Array[SimEnemy] = []
 	for e in world.enemies.values():
-		if e.alive:
-			live_enemies.append(e)
+		if not e.alive:
+			continue
+		if observer != null and eye.distance_squared_to(e.pos) > cull_sq:
+			continue
+		live_enemies.append(e)
 	b.put_u16(mini(live_enemies.size(), 65535))
 	for e in live_enemies:
 		b.put_u32(e.id)
@@ -57,6 +81,9 @@ static func encode_snapshot(world: SimWorld,
 		b.put_u8(clampi(e.def.visual, 0, 255))
 
 	var has_boss := world.boss != null and world.boss.alive
+	if has_boss and observer != null \
+			and eye.distance_squared_to(world.boss.pos) > cull_sq:
+		has_boss = false
 	b.put_u8(1 if has_boss else 0)
 	if has_boss:
 		b.put_u32(world.boss.id)

src/net/server_runtime.gd

@@ -41,9 +41,13 @@ func _physics_process(_delta: float) -> void:
 		inst.step()
 		_dispatch_events(inst)
 		if send_snapshot and not inst.peers.is_empty():
-			var snap := NetCodec.encode_snapshot(inst.world, inst.exit_countdown_seconds())
+			# Encoded per peer, not once and broadcast: each player is told only
+			# about actors near them. Fog alone was hiding enemies the client
+			# had already been handed, which is no defence against a modified
+			# client at all.
 			for peer in inst.peers:
-				Net.send_snapshot(peer, snap)
+				Net.send_snapshot(peer, NetCodec.encode_snapshot(
+					inst.world, inst.exit_countdown_seconds(), peer))
 				_stream_map(peer, inst)
 		if inst.kind != Protocol.InstanceKind.DUNGEON:
 			continue
@@ -83,9 +87,25 @@ func _dispatch_events(inst: Instance) -> void:
 			_:
 				pass
 
-	var payload := NetCodec.encode_events(inst.world.tick, events)
+	# Bullet spawns are scoped per peer too; everything else (hits, deaths,
+	# phase changes) is low volume and mostly concerns the recipient, so it goes
+	# to everyone. Despawns are deliberately NOT filtered -- a client that was
+	# told about a bullet must always be told it died, or it keeps a phantom.
+	var shared: Array[Dictionary] = []
+	var spawns: Array[Dictionary] = []
+	for ev in events:
+		if int(ev["t"]) == SimEvent.Type.BULLET_SPAWN:
+			spawns.append(ev)
+		else:
+			shared.append(ev)
+
 	for peer in inst.peers:
-		Net.send_events(peer, payload)
+		var for_peer := shared
+		if not spawns.is_empty():
+			for_peer = shared + _spawns_near(inst, peer, spawns)
+		if for_peer.is_empty():
+			continue
+		Net.send_events(peer, NetCodec.encode_events(inst.world.tick, for_peer))
 
 	for peer in to_lobby:
 		_send_to_lobby(peer)
@@ -184,11 +204,11 @@ func _place(peer_id: int, inst: Instance) -> void:
 	_stream_map(peer_id, inst)
 	# A player arriving mid-fight has no idea what is already in the air, so
 	# replay the live bullets as spawn events before the next snapshot lands.
-	var backlog := _live_bullet_events(inst.world)
+	var backlog := _live_bullet_events(inst.world, peer_id)
 	if not backlog.is_empty():
 		Net.send_events(peer_id, NetCodec.encode_events(inst.world.tick, backlog))
 	Net.send_snapshot(peer_id, NetCodec.encode_snapshot(inst.world,
-		inst.exit_countdown_seconds()))
+		inst.exit_countdown_seconds(), peer_id))
 
 
 func _transfer(peer_id: int, to: Instance) -> void:
@@ -290,11 +310,31 @@ func _broadcast_roster() -> void:
 			Net.send_roster(peer, payload)
 
 
-func _live_bullet_events(world: SimWorld) -> Array[Dictionary]:
+## Bullet spawns within this peer's interest radius. See
+## SimConfig.BULLET_INTEREST_RADIUS for why this radius is much wider than the
+## one used for actors.
+func _spawns_near(inst: Instance, peer_id: int, spawns: Array[Dictionary]) -> Array[Dictionary]:
+	var p: SimPlayer = inst.world.players.get(peer_id)
+	if p == null:
+		return spawns
+	var cull_sq := SimConfig.BULLET_INTEREST_RADIUS * SimConfig.BULLET_INTEREST_RADIUS
 	var out: Array[Dictionary] = []
+	for ev in spawns:
+		if p.pos.distance_squared_to(ev["pos"]) <= cull_sq:
+			out.append(ev)
+	return out
+
+
+func _live_bullet_events(world: SimWorld, for_peer: int = 0) -> Array[Dictionary]:
+	var out: Array[Dictionary] = []
+	var observer: SimPlayer = world.players.get(for_peer) if for_peer != 0 else null
+	var cull_sq := SimConfig.BULLET_INTEREST_RADIUS * SimConfig.BULLET_INTEREST_RADIUS
 	for i in world.pool.high_water:
 		if world.pool.alive[i] == 0:
 			continue
+		if observer != null \
+				and observer.pos.distance_squared_to(world.pool.pos[i]) > cull_sq:
+			continue
 		out.append({
 			"t": SimEvent.Type.BULLET_SPAWN,
 			"uid": world.pool.uid[i],