chore: prepare 0.9.0dev4 release

[rlippmann]

What changed

• Completed a documentation pass across src/context_compiler, adding missing public API docstrings and updating stale documentation after recent architecture changes.
• Added Ruff enforcement for public API documentation rules (D100-D103) scoped to the supported library surface.
• Added intent-focused comments to selected examples to clarify architectural patterns and avoid misleading integration assumptions.
• Bumped the development version to 0.9.0dev4.

Why

• Public APIs now have documented contracts that match current behavior.
• Ruff enforcement prevents future drift in library documentation.
• Examples now better communicate the intended architecture, especially around state ownership, preview semantics, and host integration boundaries.

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)