feat: GPU optimization — lighting, viewport CA, benchmark mode, 531 FPS

- Resolution: 8x8 world cells, 2x2 UI cells (UI_SCALE=4)
- GPU lighting: vertex-shader computed, light source list buffer (max 64)
  instead of O(N×R²) grid scan, O(N×S) per cell
- Viewport-aware CA: iterate only active chunks, not all 250×250
- Flat array entity/item/shadow maps instead of HashMaps
- Flat 128-entry ASCII atlas array instead of HashMap lookup
- Partial grid upload: viewport + 30-cell margin only
- Pre-allocated viewport arrays in renderer structs (zero alloc/frame)
- Skip CPU lighting for GPU modes (pass None)
- Benchmark mode: --mode benchmark with per-subsystem timing
- GpuLightSource struct, light_count in push constants
- gather_sources_in_range() for viewport-scoped source gathering

Benchmark (600 ticks, release):
  Graphics: 531 FPS (was 386, +38%), render 1013us (was 1699us, -40%)
  ASCII:    402 FPS (was 313, +28%), render 1502us (was 2346us, -36%)

All 171 tests + 14 scenarios pass.

tests/inventory.rs

@@ -0,0 +1,102 @@
+use verbatim::ai::AiAction;
+use verbatim::ai::GameSession;
+use verbatim::entity::{ItemManager, ItemType};
+
+fn setup() -> GameSession {
+    let mut s = GameSession::new_seeded(42);
+    s.init_empty();
+    s.clear_area(90, 90, 50, 50);
+    s
+}
+
+#[test]
+fn item_picked_up_when_near_player() {
+    let mut s = setup();
+    let pos = s.get_player().unwrap().pos;
+    let x = pos[0] as i32;
+    let y = pos[1] as i32;
+    s.game.items.spawn(ItemType::Sword, x, y);
+    assert_eq!(s.game.player.inventory.len(), 0);
+    s.step(1);
+    assert_eq!(s.game.player.inventory.len(), 1);
+    assert_eq!(s.game.player.inventory[0].typ, ItemType::Sword);
+}
+
+#[test]
+fn equipping_weapon_adds_damage_bonus() {
+    let mut s = setup();
+    let pos = s.get_player().unwrap().pos;
+    s.game
+        .items
+        .spawn(ItemType::Sword, pos[0] as i32, pos[1] as i32);
+    s.step(1);
+    s.game.use_item(0);
+    assert_eq!(s.game.player.weapon.as_ref().unwrap().typ, ItemType::Sword);
+    let bonus = s.game.player.weapon.as_ref().unwrap().damage_bonus();
+    assert!(bonus > 0.0, "equipped weapon should provide damage bonus");
+}
+
+#[test]
+fn equipping_armor_reduces_contact_damage() {
+    let mut s = setup();
+    s.perform_action(&AiAction::Spawn {
+        kind: "goblin".into(),
+        x: 120.0,
+        y: 120.0,
+    });
+    let pos = s.get_player().unwrap().pos;
+    s.game
+        .items
+        .spawn(ItemType::PlateArmor, pos[0] as i32, pos[1] as i32);
+    s.step(1);
+    s.game.use_item(0);
+    let armor = s.game.player.armor.as_ref().unwrap().armor_bonus();
+    assert!(armor > 0.0, "plate armor should provide armor bonus");
+}
+
+#[test]
+fn consumable_heals_player() {
+    let mut s = setup();
+    let pos = s.get_player().unwrap().pos;
+    s.game
+        .items
+        .spawn(ItemType::HealthPotion, pos[0] as i32, pos[1] as i32);
+    s.step(1);
+    let id = s.get_player().unwrap().id;
+    s.perform_action(&AiAction::DamageEntity { id, amount: 50.0 });
+    let health_before = s.get_player().unwrap().health;
+    s.game.use_item(0);
+    let health_after = s.get_player().unwrap().health;
+    assert!(health_after > health_before, "health potion should heal");
+}
+
+#[test]
+fn dropped_item_returns_to_world() {
+    let mut s = setup();
+    let pos = s.get_player().unwrap().pos;
+    s.game
+        .items
+        .spawn(ItemType::Dagger, pos[0] as i32, pos[1] as i32);
+    s.step(1);
+    s.game.drop_item(0);
+    assert_eq!(s.game.player.inventory.len(), 0);
+    let count = s
+        .game
+        .items
+        .all()
+        .iter()
+        .filter(|i| i.typ == ItemType::Dagger)
+        .count();
+    assert_eq!(count, 1, "dropped item should exist in world");
+}
+
+#[test]
+fn item_manager_spawns_and_removes() {
+    let mut mgr = ItemManager::new();
+    let id = mgr.spawn(ItemType::Food, 100, 100);
+    assert_eq!(id, 0);
+    assert_eq!(mgr.all().len(), 1);
+    let removed = mgr.remove_at(100, 100);
+    assert!(removed.is_some());
+    assert_eq!(mgr.all().len(), 0);
+}