refactor(control_tree): include reads in StepTreeContract signature

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

src/mir/control_tree/normalized_shadow/builder.rs

@@ -510,6 +510,7 @@ mod tests {
             contract: StepTreeContract {
                 exits: Default::default(),
                 writes: Default::default(),
+                reads: Default::default(), // Phase 124
                 required_caps: Default::default(),
                 cond_sig: Default::default(),
             },
@@ -532,6 +533,7 @@ mod tests {
             contract: StepTreeContract {
                 exits: Default::default(),
                 writes: Default::default(),
+                reads: Default::default(), // Phase 124
                 required_caps: Default::default(),
                 cond_sig: Default::default(),
             },