feat: add /type-struct skill for struct identification and typing

Formalizes the process of turning raw pointer arithmetic into typed
Rust structs: identify → map fields → create struct → update types →
replace pointer math → propagate → verify.

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

.claude/skills/type-struct/SKILL.md

@@ -0,0 +1,82 @@
+---
+name: type-struct
+description: Identify a class/struct from raw pointer usage, create a typed Rust struct, and propagate type information to eliminate pointer arithmetic. Works with Ghidra for reverse engineering context.
+---
+
+# Type a Struct
+
+Turn raw pointer arithmetic (`base.add(0x1234)`, `wb(0x878095 + idx * 0xD7B, ...)`) into typed Rust struct field access.
+
+## Process
+
+### 1. Identify the struct
+
+Find a pattern: repeated stride arithmetic, consistent base+offset reads/writes, or a Ghidra-visible vtable.
+
+- Grep for the stride/offset pattern across the codebase to find all access sites
+- Note the allocation size (from `wa_malloc`, `wa_malloc_zeroed`, or CRT malloc calls)
+- Check Ghidra for vtable assignments, constructor calls, and inheritance (xrefs to the constructor show all callers and usage contexts)
+- Determine: is this a new struct, or does it belong in an existing one (e.g., fields in DDGame, GameInfo)?
+
+### 2. Map the field layout
+
+For each offset accessed in the code, record:
+
+| Offset | Size | Type | Name | How used |
+|--------|------|------|------|----------|
+
+Compute offsets relative to the struct base. Verify with:
+- Allocation size = total struct size
+- `RET imm16` on the constructor confirms param count
+- Ghidra decompilation of the constructor shows field initialization order
+- Cross-reference with thirdparty RE sources (wkJellyWorm, WormKit) if available
+
+### 3. Create the Rust struct
+
+- Place in the appropriate module (`engine/`, `frontend/`, `task/`, `audio/`, etc.)
+- Use `#[repr(C)]` for ABI compatibility
+- Add `const _: () = assert!(core::mem::size_of::<T>() == SIZE);` to verify total size
+- Name known fields descriptively; use `_unknown_XXXX: [u8; N]` for gaps
+- If the struct has a vtable, create a companion `FooVtable` struct with typed function pointers for known slots and `usize` for unknown slots
+- Register the module in the parent `mod.rs` and re-export if appropriate
+
+### 4. Update variable and argument types
+
+- Replace `u32` / `*mut u8` parameters with `*mut NewStruct` in function signatures
+- This is ABI-compatible on i686 (all 32-bit pointers) so no calling convention changes needed
+- Update both Rust functions AND FFI `extern` function pointer types (stdcall, thiscall, cdecl)
+- For naked asm bridges, parameter types can be changed without affecting the assembly
+
+### 5. Replace pointer arithmetic with field access
+
+- `*(base.add(0x1234) as *mut u32) = val` → `(*ptr).field_name = val`
+- `wb(CONST + idx * STRIDE, val)` → `array[idx].field = val`
+- For vtable calls: `*(vtable as *const u32).add(N)` → `(*(*ptr).vtable).method`
+- Use `vcall!` macro for one-liner vtable dispatch where appropriate
+
+### 6. Propagate types
+
+- Update callers to pass typed pointers instead of `u32`
+- Remove now-unnecessary `_OFF` address constants from `address.rs` (offsets are now struct fields)
+- Rename address constants if the struct name changed (e.g., `MAP_CLASS_*` → `MAP_VIEW_*`)
+- Update Ghidra labels to match the new names
+
+### 7. Verify
+
+- `cargo build --release -p openwa-wormkit` — must compile clean, zero warnings
+- Run `/replay-test` (headful + headless) to verify runtime correctness
+- Check that `sizeof` assertion passes (struct size matches allocation size)
+
+## Naming conventions
+
+- Struct names: `PascalCase`, matching WA class names where known (e.g., `DDGame`, `CTaskWorm`, `MapView`)
+- Unknown fields: `_unknown_XXXX` where XXXX is the hex offset
+- Vtable structs: `FooVtable` companion to `Foo`
+- Address constants: `STRUCT_NAME_FUNCTION` (e.g., `MAP_VIEW_CONSTRUCTOR`)
+- Module placement: `engine/` for core game objects, `frontend/` for UI/CWnd-derived, `task/` for CTask hierarchy, `audio/` for sound, `display/` for graphics
+
+## When NOT to create a struct
+
+- If offsets use inconsistent strides (e.g., player data with mixed 0x78/0x3C/0x1E strides) — these are interleaved arrays, not a single struct
+- If the "struct" is actually a function's stack frame (local variables)
+- If only 1-2 fields are known and the struct is huge — use raw offsets with named constants until more fields are discovered