refactor(simulation): simplify vision cone to forward-only 120° arc

Remove the peripheral sector (100° half-angle, reduced range) and blind
spot classification. The server now sends only tiles within the 120°
forward cone; the client renders previously-explored tiles behind the
player with a light fog overlay instead.

This eliminates complexity in both the cone classifier and the snapshot
protocol while preserving the core information asymmetry — you still
can't see behind you, and the monologue system (D-016) still bridges
the perceptual gap.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/perception/observer/tests.rs

@@ -590,10 +590,10 @@ fn npc_behind_wall_excluded_from_multi_entity_snapshot() {
     ));
     // NPC 1: behind wall (should be hidden)
     world.spawn((crate::npc::Npc, TilePosition::new(16, 13, 0)));
-    // NPC 2: to the side, no wall (should be visible)
+    // NPC 2: NW diagonal, in forward cone (should be visible)
     world.spawn((crate::npc::Npc, TilePosition::new(14, 14, 0)));
-    // NPC 3: also visible
-    world.spawn((crate::npc::Npc, TilePosition::new(18, 15, 0)));
+    // NPC 3: NE diagonal, in forward cone (should be visible)
+    world.spawn((crate::npc::Npc, TilePosition::new(18, 14, 0)));
 
     run_observer_pipeline(&mut world);
 
@@ -1911,7 +1911,7 @@ fn equidistant_npcs_produce_stable_snapshot_ordering() {
     let mut world = setup_world(32, 32);
     let mut registry = EntityRegistry::new(0);
 
-    // Three NPCs equidistant from observer at (16,16) — all 2 tiles away.
+    // Three NPCs equidistant from observer at (16,16) — all within forward cone.
     // Spawn order: npc_a, npc_b, npc_c → ascending stable_ids.
     let npc_a = world
         .spawn((crate::npc::Npc, TilePosition::new(16, 14, 0)))
@@ -1919,12 +1919,12 @@ fn equidistant_npcs_produce_stable_snapshot_ordering() {
     let npc_a_sid = registry.register(npc_a);
 
     let npc_b = world
-        .spawn((crate::npc::Npc, TilePosition::new(14, 16, 0)))
+        .spawn((crate::npc::Npc, TilePosition::new(14, 14, 0)))
         .id();
     let npc_b_sid = registry.register(npc_b);
 
     let npc_c = world
-        .spawn((crate::npc::Npc, TilePosition::new(18, 16, 0)))
+        .spawn((crate::npc::Npc, TilePosition::new(18, 14, 0)))
         .id();
     let npc_c_sid = registry.register(npc_c);