docs: define API stability and deprecation policy #116
Description
Problem
Downstream apps need predictable upgrade paths as Miaou evolves quickly.
Proposal
Document a clear stability contract and release policy.
Scope
- Add docs/stability-policy.md covering:
- semantic versioning guarantees
- stable vs experimental module criteria
- deprecation lifecycle and minimum grace period
- breaking-change communication requirements
- Add a PR checklist item requiring migration notes for breaking changes.
Acceptance criteria
- Policy is referenced from CONTRIBUTING.md.
- Breaking changes in changelog follow this policy.
- Maintainers have a standard process for marking APIs experimental or stable.