Phase 124-P3:
- Add env: BTreeMap<String, ValueId> from writes in lower_if_only_to_normalized
- Pass env and contract to lower_return_from_tree, lower_if_node, lower_return_value
- Implement Return(Variable) support:
- If variable in env (writes): Ret(Some(vid))
- If variable not in env: Err (out of scope, phase124 error)
- Add phase124 errors to Ok(None) conversion (dev-only compatibility)
- Add unit test: test_return_variable_from_env (verifies Ret(Some(ValueId(1))))
- All 1159 tests PASS
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
Phase 122 P1: StepTree→Normalized JoinModule generation
- Generate env layout from writes (SSOT, deterministic)
- Minimal implementation: main function + Ret only
- Full If/Assign/Return lowering in future P2-P4
- Dev-only: no behavior change to existing path
