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/projectiles.rs

@@ -0,0 +1,188 @@
+use verbatim::ai::{AiAction, GameSession};
+
+fn setup() -> GameSession {
+    let mut s = GameSession::new_seeded(42);
+    s.init_empty();
+    s.clear_area(80, 80, 80, 80);
+    s.perform_action(&AiAction::FillRect {
+        x: 80,
+        y: 130,
+        w: 80,
+        h: 5,
+        material: "stone".into(),
+    });
+    s
+}
+
+#[test]
+fn projectile_travels_and_deals_damage() {
+    let mut s = setup();
+    let player_pos = s.get_player().unwrap().pos;
+    s.perform_action(&AiAction::Spawn {
+        kind: "goblin".into(),
+        x: player_pos[0] + 15.0,
+        y: player_pos[1],
+    });
+    s.step(10);
+    let goblin = s
+        .get_entities()
+        .into_iter()
+        .find(|e| e.kind == "Goblin")
+        .unwrap();
+    let hp_before = goblin.health;
+    s.perform_action(&AiAction::Shoot {
+        dir_x: 1.0,
+        dir_y: 0.0,
+    });
+    s.step(10);
+    let goblin_after = s
+        .get_entities()
+        .into_iter()
+        .find(|e| e.kind == "Goblin")
+        .unwrap();
+    assert!(
+        goblin_after.health < hp_before,
+        "goblin should take projectile damage: {} -> {}",
+        hp_before,
+        goblin_after.health
+    );
+}
+
+#[test]
+fn projectile_stops_on_solid_cell() {
+    let mut s = setup();
+    let player_pos = s.get_player().unwrap().pos;
+    let wall_x = player_pos[0] as i32 + 8;
+    s.perform_action(&AiAction::FillRect {
+        x: wall_x,
+        y: player_pos[1] as i32 - 2,
+        w: 4,
+        h: 4,
+        material: "stone".into(),
+    });
+    s.perform_action(&AiAction::Shoot {
+        dir_x: 1.0,
+        dir_y: 0.0,
+    });
+    s.step(10);
+    let cell = s.get_cell(wall_x, player_pos[1] as i32);
+    assert_eq!(
+        cell.material, "stone",
+        "projectile should not destroy stone wall"
+    );
+    let state = s.get_state();
+    let projectile_count = state
+        .entities
+        .iter()
+        .filter(|e| e.kind == "Projectile")
+        .count();
+    assert_eq!(
+        projectile_count, 0,
+        "projectile should be destroyed after hitting wall"
+    );
+}
+
+#[test]
+fn fireball_ignites_wood() {
+    let mut s = setup();
+    let player_pos = s.get_player().unwrap().pos;
+    s.perform_action(&AiAction::FillRect {
+        x: player_pos[0] as i32 + 5,
+        y: player_pos[1] as i32 - 2,
+        w: 6,
+        h: 4,
+        material: "wood".into(),
+    });
+    s.perform_action(&AiAction::ToggleFireball);
+    s.perform_action(&AiAction::Shoot {
+        dir_x: 1.0,
+        dir_y: 0.0,
+    });
+    s.step(15);
+    let fire_count = s.count_material_in_region(
+        player_pos[0] as i32 + 4,
+        player_pos[1] as i32 - 3,
+        10,
+        8,
+        "fire",
+    );
+    assert!(
+        fire_count > 0,
+        "fireball should ignite wood, got {} fire cells",
+        fire_count
+    );
+}
+
+#[test]
+fn corpse_decomposes_into_flesh_cells() {
+    let mut s = setup();
+    s.perform_action(&AiAction::FillRect {
+        x: 100,
+        y: 130,
+        w: 40,
+        h: 5,
+        material: "stone".into(),
+    });
+    s.perform_action(&AiAction::FillRect {
+        x: 100,
+        y: 100,
+        w: 1,
+        h: 30,
+        material: "stone".into(),
+    });
+    s.perform_action(&AiAction::FillRect {
+        x: 139,
+        y: 100,
+        w: 1,
+        h: 30,
+        material: "stone".into(),
+    });
+    s.perform_action(&AiAction::Spawn {
+        kind: "goblin".into(),
+        x: 110.0,
+        y: 120.0,
+    });
+    s.step(10);
+    let goblin = s
+        .get_entities()
+        .into_iter()
+        .find(|e| e.kind == "Goblin")
+        .unwrap();
+    s.perform_action(&AiAction::DamageEntity {
+        id: goblin.id,
+        amount: 100.0,
+    });
+    s.step(100);
+    let corpse = s
+        .get_entities()
+        .into_iter()
+        .find(|e| e.kind == "Corpse")
+        .unwrap();
+    let _pos = corpse.pos;
+    let after = s.count_material_in_region(100, 120, 40, 20, "flesh");
+    assert!(
+        after > 0,
+        "corpse should decompose into flesh cells, got {}",
+        after
+    );
+}
+
+#[test]
+fn ui_hud_shows_player_hp() {
+    let mut s = setup();
+    let mut ui = verbatim::ui::UiLayer::new();
+    let player = s.game.entities.all()[0].clone();
+    ui.draw_hud(
+        80,
+        25,
+        Some(&player),
+        s.tick(),
+        verbatim::world::cell::MaterialId::Sand,
+        0,
+        0,
+        1,
+        &s.game.player,
+        60.0,
+    );
+    assert!(ui.get(0, 8).is_some(), "HUD should draw HP label");
+}