Add TODO.md with next implementation steps

Document the current completed state and outline the next phase:
- Mock code generator implementation
- Test project with C linkage and function coverage
- Golden file regression testing
- Multi-header support

Provides clear roadmap based on TEST_HARNESS_PLAN_V2.md with
time estimates and prioritization.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

lib/sdl3/parser/TODO.md

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+# SDL3 Parser - Next Steps
+
+## Current Status ✅
+
+The parser is **complete and functional** with:
+- All C declaration types supported (opaque, enum, struct, flags, functions)
+- Proper naming conventions implemented ("first underscore" rule)
+- Memory leak free (validated with GPA)
+- 18+ unit tests, all passing
+- Comprehensive documentation under `docs/`
+- Successfully parses SDL_gpu.h (169 declarations)
+
+## Next Implementation Phase
+
+Based on `TEST_HARNESS_PLAN_V2.md`, the next logical steps are:
+
+### 1. Implement Mock Code Generator (~3 hours)
+
+Create `mock_codegen.zig` to generate C mock implementations when `--mocks` flag is passed:
+
+```bash
+zig build run -- SDL_gpu.h --mocks > gpu_mocks.c
+```
+
+**Tasks:**
+- [ ] Add `--mocks` CLI flag parsing in `parser.zig`
+- [ ] Create `mock_codegen.zig` module
+- [ ] Generate stub C functions that return null/0/default values
+- [ ] Generate C header declarations
+- [ ] Add unit tests for mock generation
+
+### 2. Create Test Project (~4 hours)
+
+Build `test_project/` with complete compilation and linkage testing:
+
+**Tasks:**
+- [ ] Create `test_project/` directory structure
+- [ ] Set up `build.zig` to compile C mocks into static library
+- [ ] Create `c.zig` that links against mock library
+- [ ] Generate Zig bindings from SDL_gpu.h
+- [ ] Create `test_main.zig` that calls all generated functions
+- [ ] Add assertions to verify function calls work
+- [ ] Integrate into main `build.zig` as `zig build test-project`
+
+### 3. Add Golden File Testing (~2 hours)
+
+Implement regression testing to catch unintended output changes:
+
+**Tasks:**
+- [ ] Generate golden reference file from current parser output
+- [ ] Create comparison test in `test_project/`
+- [ ] Add diff reporting when output changes
+- [ ] Add `--update-golden` flag to accept new output
+
+### 4. Multi-Header Support (~2 hours)
+
+Test parser on additional SDL3 headers:
+
+**Tasks:**
+- [ ] Test with `SDL_video.h`
+- [ ] Test with `SDL_audio.h`
+- [ ] Test with `SDL_events.h`
+- [ ] Document any new patterns discovered
+- [ ] Add pattern-specific tests if needed
+
+## Future Enhancements
+
+### Nice to Have
+- [ ] Performance benchmarking and profiling
+- [ ] Batch processing script for multiple headers
+- [ ] CI/CD integration for automated testing
+- [ ] Fuzz testing with random C headers
+- [ ] Support for function pointer types (basic support exists)
+- [ ] Support for union types
+- [ ] Support for complex macros (beyond simple #define)
+
+### Documentation
+- [ ] Add examples of using generated bindings in real projects
+- [ ] Create video/tutorial for using the parser
+- [ ] Document known limitations and unsupported patterns
+
+## Time Estimate
+
+**Test Harness Implementation**: ~10 hours total
+- Mock generator: 3 hours
+- Test project: 4 hours
+- Golden file testing: 2 hours
+- Multi-header testing: 1 hour
+
+## Getting Started
+
+To begin the next phase:
+
+1. Read `TEST_HARNESS_PLAN_V2.md` for complete design
+2. Start with mock code generator implementation
+3. Use test-driven development (write tests first)
+4. Run `zig build test` frequently to verify changes
+5. Update this TODO.md as tasks are completed
+
+## Questions/Decisions Needed
+
+- Should mocks return null/zero or track call counts?
+- Should test project test all functions or just a subset?
+- What's the acceptable diff threshold for golden file testing?
+- Should we support C++ headers in the future?
+
+---
+
+Last updated: 2026-01-21
+Parser version: Working, all tests passing