refactor: separate subsystem API boundaries

[rlippmann]

What changed

• removed grammar re-exports from the root package so context_compiler.grammar is the authoritative public grammar import path
• moved auditability APIs behind a dedicated context_compiler.audit public module boundary
• updated the root and audit public API conformance artifacts, boundary tests, and related documentation/examples
• bumped the package dev release version in pyproject.toml and refreshed uv.lock

Why

• the 0.9 release line is stabilizing public boundaries, and the root package should represent the primary runtime API rather than overlapping subsystem contracts
• grammar already had an independent public contract, and auditability now has its own dedicated public module surface
• the dev release metadata needed to reflect the updated branch state for publishing

Checklist

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