feat: add public directive grammar foundation

[rlippmann]

What changed

• Added the public context_compiler.grammar submodule with DirectiveKind, ValidatedDirective, validate_directive(), is_canonical_directive(), and render_directive().
• Added a private immutable grammar registry that drives canonical validation, rendering, operand requirements, and future parser reuse without exposing engine authority semantics.
• Added focused unit tests, public API contract coverage, language-neutral grammar behavior fixtures, and concise API documentation.
• Preserved the existing engine parser and deferred engine migration, consumer cleanup, broader read-only conformance work, and RLock hardening to follow-up PRs.

Why

• Establish a narrow, stable grammar boundary that downstream consumers such as the directive drafter and example integrations can reuse without duplicating canonical syntax.
• Separate syntax ownership from engine-owned authority, clarification, mutation, and state semantics.
• Create the foundation for migrating the engine parser to a single grammar source of truth in a focused follow-up PR.

Checklist

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