fix: export public policy typing

[rlippmann]

What changed

• exported PolicyValue and DecisionKind from the root context_compiler public API
• updated the public API conformance fixture to reflect the intended root export surface
• added regression coverage that verifies the public annotation dependency imports work from the package root
• bumped the package dev release version in pyproject.toml and refreshed uv.lock

Why

• PolicyValue is used in exported API annotations, and DecisionKind is referenced by the exported Decision type, so the root public API needed to expose those symbols consistently
• the public API contract fixture needed to match the intended root export surface before the next dev release
• the package metadata and lockfile needed a new dev release version for publishing the updated branch state

Checklist

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