0.6.16 — preprocessor naming and fixture taxonomy cleanup

[rlippmann]

What changed

• Renamed the preprocessor conformance fixture directory from tests/fixtures/precompiler/ to tests/fixtures/preprocessor/ and updated the conformance harness path.
• Migrated preprocessor harness naming in tests/docs to prefer “preprocessor” terminology.
• Added compatibility-preserving API naming in experimental/preprocessor/output_validation.py:
  • preferred: parse_preprocessor_output(...)
  • compatibility alias retained: parse_precompiler_output(...)
• Added a preferred CLI flag --with-preprocessor while preserving --with-precompiler as a compatibility alias (shared destination/behavior).
• Updated docs and examples to prefer preprocessor terminology and clarified fixture-suite taxonomy in tests/fixtures/README.md:
  • conformance/ (engine cross-language contract)
  • engine-regression/structured/ (deterministic engine regression/checkpoint snapshots)
  • preprocessor/ (heuristic/validation fixtures)
• Kept compatibility shims/paths where needed to avoid breaking existing imports/usages.
• Release/version housekeeping for 0.6.16 included in this branch.

Why

• Align naming and user-facing terminology ahead of TS 0.6 work.
• Reduce ambiguity between engine conformance fixtures and preprocessor fixtures.
• Preserve backward compatibility in 0.6.x while introducing preferred names.