Phase 124-P2:
- Add reads: BTreeSet<String> to StepTreeContract
- Include reads in signature_basis_string (format: ...;reads=...;...)
- Update from_facts() to copy reads from StepTreeFacts
- Update all StepTreeContract construction sites (builder.rs, parity.rs tests)
- Update test expected signature to include reads field
- Maintains determinism: BTreeSet guarantees stable iteration order
