chore: sync Python 0.6.19 conformance and preprocessor fixtures

[rlippmann]

Summary

• Sync conformance fixtures to Python v0.6.19.
• Add synced preprocessor fixture corpus (including public-api-v1.json) from Python v0.6.19.
• Update CI fixture pin (PY_FIXTURE_REF) to the Python v0.6.19 commit for deterministic drift checks.

Why

• Keep TS fixture contracts aligned with the latest published Python source-of-truth.
• Pick up fixture contract expansion added since prior TS sync (checkpoint/state-json coverage, pending punctuation tokens, expanded preprocessor validator fixtures).
• Ensure CI verifies against the same Python fixture revision used for sync.

Scope checklist

• Diff is minimal and scoped to the requested behavior
• Python/spec parity is preserved (or mismatch is explicitly documented)
• Fixture semantics are respected (exact string vs non-empty null rule)
• No unrelated refactors
• Relevant tests were run

Notes

• Commands run:
  • npm test
  • FIXTURES_SOURCE=/tmp/context-compiler-0.6.19-fixtures/conformance/tests/fixtures/conformance npm run fixtures:check
  • diff -ru /tmp/context-compiler-0.6.19-fixtures/preprocessor/tests/fixtures/preprocessor tests/fixtures/preprocessor
• This PR is sync/audit only; no engine or preprocessor implementation behavior changes are included.