## Description
Extracts canonical memory-tool definition byte serialization from
`headroom.proxy.helpers` into a focused pure module. The existing helper
function remains as a compatibility wrapper for sticky memory tool and
CCR replay code.
## Type of Change
- [ ] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [ ] Performance improvement
- [x] Code refactoring (no functional changes)
## Changes Made
- Added `headroom.proxy.tool_definition_serialization` for deterministic
compact UTF-8 tool definition serialization.
- Kept `helpers.serialize_tool_definition_canonical()` as a
compatibility wrapper.
- Added direct unit tests for compact separators, Unicode preservation,
insertion-order byte stability, and parity with the existing body
canonicalizer.
## Testing
- [x] Unit tests pass (`pytest`)
- [x] Linting passes (`ruff check`)
- [x] Formatting passes (`ruff format --check`)
- [x] New tests added for new functionality
- [ ] Manual testing performed
### Test Output
```text
uv run pytest tests/test_tool_definition_serialization.py tests/test_ccr_tool_always_on.py tests/test_memory_tool_session_sticky.py tests/test_proxy_byte_faithful_forwarding.py -q
85 passed, 1 warning in 2.47s
uvx --from ruff==0.15.17 ruff check headroom/proxy/helpers.py headroom/proxy/tool_definition_serialization.py tests/test_tool_definition_serialization.py --output-format concise
All checks passed!
uvx --from ruff==0.15.17 ruff format --check headroom/proxy/helpers.py headroom/proxy/tool_definition_serialization.py tests/test_tool_definition_serialization.py
3 files already formatted
```
## Real Behavior Proof
- Environment: Windows, Python 3.13.x
- Exact command / steps: Ran direct serializer tests plus CCR always-on,
sticky memory tool, and proxy byte-faithful forwarding regression
coverage; then checked the touched files with the CI-pinned Ruff
version.
- Observed result: Serializer byte contract remains directly covered
while existing sticky replay and byte-faithful proxy behavior stay
green.
- Not tested: Full repository pytest suite locally; GitHub CI is green
for the current head.
## Review Readiness
- [x] I have performed a self-review
- [x] This PR is ready for human review
## Checklist
- [x] My code follows the project's style guidelines
- [x] I have performed a self-review of my code
- [x] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] I have updated the CHANGELOG.md if applicable
## Screenshots (if applicable)
N/A
## Additional Notes
Documentation and changelog updates are not applicable for this internal
refactor. The current head is mergeable and GitHub checks are green.
---------
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
