feat(joinir): Phase 171-fix ConditionEnv/ConditionBinding architecture

Proper HOST↔JoinIR ValueId separation for condition variables:

- Add ConditionEnv struct (name → JoinIR-local ValueId mapping)
- Add ConditionBinding struct (HOST/JoinIR ValueId pairs)
- Modify condition_to_joinir to use ConditionEnv instead of builder.variable_map
- Update Pattern2 lowerer to build ConditionEnv and ConditionBindings
- Extend JoinInlineBoundary with condition_bindings field
- Update BoundaryInjector to inject Copy instructions for condition variables

This fixes the undefined ValueId errors where HOST ValueIds were being
used directly in JoinIR instructions. Programs now execute (RC: 0),
though loop variable exit values still need Phase 172 work.

Key invariants established:
1. JoinIR uses ONLY JoinIR-local ValueIds
2. HOST↔JoinIR bridging is ONLY through JoinInlineBoundary
3. condition_to_joinir NEVER accesses builder.variable_map

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/builder/control_flow/joinir/merge/mod.rs

@@ -74,9 +74,22 @@ pub(in crate::mir::builder) fn merge_joinir_mir_blocks(
     let mut remapper = block_allocator::allocate_blocks(builder, mir_module, debug)?;
 
     // Phase 2: Collect values from all functions
-    let (used_values, value_to_func_name, function_params) =
+    let (mut used_values, value_to_func_name, function_params) =
         value_collector::collect_values(mir_module, &remapper, debug)?;
 
+    // Phase 171-fix: Add condition_bindings' join_values to used_values for remapping
+    if let Some(boundary) = boundary {
+        for binding in &boundary.condition_bindings {
+            if debug {
+                eprintln!(
+                    "[cf_loop/joinir] Phase 171-fix: Adding condition binding '{}' JoinIR {:?} to used_values",
+                    binding.name, binding.join_value
+                );
+            }
+            used_values.insert(binding.join_value);
+        }
+    }
+
     // Phase 3: Remap ValueIds
     remap_values(builder, &used_values, &mut remapper, debug)?;