chore: enforce full fixture drift checks in CI

[rlippmann]

Summary

• Updates CI to run existing fixture drift checks for all three fixture trees:
  • conformance
  • preprocessor
  • structured regression
• Keeps CI drift checks tied to the same pinned Python fixture source ref already configured in workflow env.
• Adds short local command guidance in CONTRIBUTING.md for running preprocessor and structured fixture checks, alongside conformance checks.

Why

This prevents partial fixture sync mistakes by enforcing drift checks across all portable fixture trees in CI, not only conformance.

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 validation was run (npm test for behavior changes, or docs-only/no-runtime-change)

Notes

• tests/fixtures/README.md drift remains intentionally manual for now because current drift scripts compare fixture tree directories only.

Validation details:

• FIXTURES_SOURCE=/Users/rlippmann/Source/context-compiler/tests/fixtures/conformance npm run fixtures:check: passed
• PREPROCESSOR_FIXTURES_SOURCE=/Users/rlippmann/Source/context-compiler/tests/fixtures/preprocessor npm run fixtures:preprocessor:check: passed
• STRUCTURED_FIXTURES_SOURCE=/Users/rlippmann/Source/context-compiler/tests/fixtures/engine-regression/structured npm run fixtures:structured:check: passed
• npm test: passed
• npm run build: passed