feat: add public directive decomposition

[rlippmann]

What changed

• add CanonicalDirective and decompose_directive(...) to context_compiler.grammar as a public syntax-only decomposition API
• refactor grammar validation to project from the same decomposition path so grammar has one authoritative directive interpretation
• migrate the engine parse boundary to consume the public grammar decomposition result without changing action, clarify, or state-transition semantics
• extend the public grammar API contract, fixture harness, and grammar conformance fixtures to cover decomposition, zero-operand directives, invalid inputs, and operand text preservation
• clean up stale test references to the retired private parser helper name now that decompose_directive(...) is the single grammar decomposition path
• bump the package version from 0.9.0dev2 to 0.9.0dev3 and keep uv.lock in sync
• document that decomposition exposes canonical syntax only and preserves grammar-recognized operand text rather than semantic normalization

Why

• external consumers need a stable way to inspect directive kind and operands for already-valid canonical directives
• this keeps syntax inspection in the grammar layer while preserving the existing boundary where engine semantics, clarification, and state transitions remain private
• unifying validation and engine parsing around the same grammar representation reduces drift between the public grammar surface and internal consumers
• the follow-up cleanup removes stale references to the retired private helper so tests describe current behavior rather than historical implementation details
• the version bump makes the expanded public grammar surface explicit in the package metadata for this release candidate

Checklist

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