refactor: use TaskStateMachine struct for sprite width/height reads

Replace *(sprite.add(0x14)) and *(sprite.add(0x18)) with typed
TaskStateMachine field access (.width, .height) in the arrow
collision region computation.

Remaining pointer arithmetic candidates in ddgame.rs:
- GfxHandler +0x198 (file handle) — needs GfxHandler struct (0x19C bytes)
- DDDisplay +0x3120/+0x3534/+0x312C (palette range) — DDDisplay fields
- PCLandscape +0xB34/+0xB38 (land/water layer ptrs) — PCLandscape fields
- DDGame raw fields (0x468, 0x7308, 0x777C, 0x7780) — unnamed struct fields

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/openwa-core/src/engine/ddgame.rs

@@ -1028,8 +1028,9 @@ unsafe fn init_graphics_and_resources(
 
             // Calculate collision region dimensions from sprite
             if !sprite.is_null() {
-                let sprite_w = *(sprite.add(0x14) as *const i32);
-                let sprite_h = *(sprite.add(0x18) as *const i32);
+                let tsm = &*(sprite as *const TaskStateMachine);
+                let sprite_w = tsm.width as i32;
+                let sprite_h = tsm.height as i32;
                 let half_w = (sprite_w / 2 - 10).max(0);
                 let half_h = (sprite_h / 2 - 10).max(0);