test: expand preprocessor property coverage

[rlippmann]

Summary

• expand Hypothesis coverage for heuristic preprocessing and output validation
• add generated coverage for compound directive detection across directive families and separators
• exercise lexical-boundary and canonical-looking substring cases to guard against false compound detection
• add validator properties for compound candidate rejection and structured non-directive output contracts
• add parse/validate round-trip properties for directive and non-directive outcomes

Notes

• this is a test-only change and does not modify runtime behavior
• generated coverage preserves the distinction between canonical directive starts and canonical-looking substrings inside singular payloads
• the expanded suite passes with 117 passed
• uv run pre-commit run --all-files also passes