feat(joinir/dev): Phase 126 wire available_inputs into normalized builder

- AvailableInputsCollectorBox::collect() called in lower_function_body (dev-only)
- try_lower_if_only() signature extended (accepts available_inputs)
- EnvLayout::from_contract() now uses real available_inputs (not empty stub)
- Unit tests updated (empty BTreeMap for backward compat)
- All 23 normalized_shadow tests PASS

src/mir/builder/calls/lowering.rs

@@ -203,9 +203,14 @@ impl MirBuilder {
             if dev {
                 use crate::mir::control_tree::normalized_shadow::StepTreeNormalizedShadowLowererBox;
                 use crate::mir::control_tree::normalized_shadow::parity;
+                use crate::mir::control_tree::normalized_shadow::available_inputs_collector::AvailableInputsCollectorBox;
+
+                // Phase 126: Collect available_inputs from SSOT sources
+                // Note: CapturedEnv is None for now (if-only patterns don't use CapturedEnv yet)
+                let available_inputs = AvailableInputsCollectorBox::collect(self, None);
 
                 // Try shadow lowering (if-only scope)
-                let shadow_result = StepTreeNormalizedShadowLowererBox::try_lower_if_only(&tree);
+                let shadow_result = StepTreeNormalizedShadowLowererBox::try_lower_if_only(&tree, &available_inputs);
 
                 match shadow_result {
                     Ok(Some((module, _meta))) => {