Reject compound directives and strengthen grammar validation

[rlippmann]

Summary

Strengthen the directive grammar by enforcing the single-directive invariant.

Inputs containing more than one canonical directive are now rejected through the existing public clarify decision contract. No authoritative state is mutated and no pending clarification/replacement state is created.

This PR also adds comprehensive regression coverage, conformance fixtures, property-based testing, and documentation updates.

Changes

Grammar

• Reject inputs containing multiple canonical directive starts.
• Apply the rule uniformly across all canonical directive families.
• Base detection on canonical directive starts rather than conjunctions or separators.
• Preserve current quote semantics (quotes do not create protected parsing regions).

Implementation

• Route compound directives through the existing recognized-invalid directive path.
• Preserve pending-confirmation precedence.
• Preserve existing single-directive behavior.
• Centralize canonical directive-start definitions to reduce parser/scanner drift.

Tests

• Add regression coverage across directive families, separators, quoting, pending confirmation, transcript replay, and parser/scanner alignment.
• Add focused Hypothesis property tests for lexical boundary handling.
• Update existing property tests to reflect the tightened grammar.

Conformance

• Add canonical compound-directive conformance fixtures.
• Add transcript replay conformance coverage for compound directives.

Documentation

• Document the single-directive grammar invariant.
• Document compound directives as invalid grammar reported through the existing clarify contract.
• Document current quote behavior.
• Add concise grammar examples.

Validation

• uv run pytest
• uv run pytest --cov=context_compiler --cov-report=term-missing
• uv run pre-commit run --all-files

All passed.

Closes #179