feat(control_tree): Phase 126 AvailableInputsCollectorBox

- Collect available_inputs from function params + CapturedEnv (SSOT)
- BTreeMap for deterministic order
- Box-first modularization with unit tests (5 tests PASS)
- Source priority: params > CapturedEnv
- No AST inference (only pre-computed sources)

src/mir/control_tree/normalized_shadow/mod.rs

@@ -32,6 +32,7 @@
 pub mod builder;
 pub mod contracts;
 pub mod parity;
+pub mod available_inputs_collector; // Phase 126: available_inputs SSOT
 
 pub use builder::StepTreeNormalizedShadowLowererBox;
 pub use contracts::{CapabilityCheckResult, UnsupportedCapability};