fix: make preprocessor validation output-only

[rlippmann]

Summary

• remove the source_input parameter and source-aware acceptance policy from validate_preprocessor_output(...) and parse_preprocessor_output(...)
• make validation and parsing strictly responsible for preprocessor output shape and canonical directive grammar
• remove obsolete source-aware helpers, tests, and fixtures, and update the public API contract, examples, and documentation
• preserve the prior output-only behavior without broadening normalization or repairing grammar near misses
• clarify that structural validation does not establish semantic faithfulness to the user’s original input

Notes

• this is an intentional pre-1.0 contract correction
• direct before/after auditing confirmed that the prior output-only validator/parser behavior is preserved
• known grammar near misses remain rejected, and no unintended output-only broadening or narrowing was found
• source-aware policy for unattended model-generated directives is intentionally not replaced in this PR; automatic-handoff policy and reviewed draft_directive(...) behavior are separate follow-up concerns

Closes #15 by removing source-aware semantic acceptance from output validation; unattended automatic-handoff policy remains a separate follow-up concern.