fix: resolve CI workflow parse error for fixture checks

[rlippmann]

Summary

• Replace unsupported expression-based temp paths in CI env with explicit /tmp paths.
• Preserve fixture drift validation against pinned Python source-of-truth fixtures.

Why this change

• GitHub Actions was rejecting the workflow at parse time (Unrecognized named-value: runner).
• With the parse issue fixed, CI runs again and required status checks can enforce merge gating.

Details

• PY_FIXTURE_CHECKOUT now uses /tmp/context-compiler-source.
• FIXTURES_SOURCE in the fixture drift step now uses /tmp/context-compiler-source/tests/fixtures/conformance.
• No fixture JSON or engine semantics changes in this PR.

[rlippmann]

Closing in favor of #16, which carries the same CI parse fix on a clean single-commit branch based on current main.