feat(llvm): Phase 132 - Pattern 1 exit value parity fix + Box-First refactoring

## Phase 132: Exit PHI Value Parity Fix

### Problem
Pattern 1 (Simple While) returned 0 instead of final loop variable value (3)
- VM: RC: 3 ✅ (correct)
- LLVM: Result: 0 ❌ (wrong)

### Root Cause (Two Layers)
1. **JoinIR/Boundary**: Missing exit_bindings → ExitLineReconnector not firing
2. **LLVM Python**: block_end_values snapshot dropping PHI values

### Fix
**JoinIR** (simple_while_minimal.rs):
- Jump(k_exit, [i_param]) passes exit value

**Boundary** (pattern1_minimal.rs):
- Added LoopExitBinding with carrier_name="i", role=LoopState
- Enables ExitLineReconnector to update variable_map

**LLVM** (block_lower.py):
- Use predeclared_ret_phis for reliable PHI filtering
- Protect builder.vmap PHIs from overwrites (SSOT principle)

### Result
- ✅ VM: RC: 3
- ✅ LLVM: Result: 3
- ✅ VM/LLVM parity achieved

## Phase 132-Post: Box-First Refactoring

### Rust Side
**JoinModule::require_function()** (mod.rs):
- Encapsulate function search logic
- 10 lines → 1 line (90% reduction)
- Reusable for Pattern 2-5

### Python Side
**PhiManager Box** (phi_manager.py - new):
- Centralized PHI lifecycle management
- 47 lines → 8 lines (83% reduction)
- SSOT: builder.vmap owns PHIs
- Fail-Fast: No silent overwrites

**Integration**:
- LLVMBuilder: Added phi_manager
- block_lower.py: Delegated to PhiManager
- tagging.py: Register PHIs with manager

### Documentation
**New Files**:
- docs/development/architecture/exit-phi-design.md
- docs/development/current/main/investigations/phase132-llvm-exit-phi-wrong-result.md
- docs/development/current/main/phases/phase-132/

**Updated**:
- docs/development/current/main/10-Now.md
- docs/development/current/main/phase131-3-llvm-lowering-inventory.md

### Design Principles
- Box-First: Logic encapsulated in classes/methods
- SSOT: Single Source of Truth (builder.vmap for PHIs)
- Fail-Fast: Early explicit failures, no fallbacks
- Separation of Concerns: 3-layer architecture

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/mir/builder/control_flow/joinir/patterns/pattern1_minimal.rs

@@ -74,12 +74,32 @@ impl MirBuilder {
         // Phase 179-B: Create boundary from context
         // Phase 201: Use JoinInlineBoundaryBuilder for clean construction
         // Canonical Builder pattern - see docs/development/current/main/joinir-boundary-builder-pattern.md
+        //
+        // Phase 132: Add exit_bindings to enable ExitLineReconnector
+        // This ensures `return i` after loop returns the final value (3) instead of initial (0)
         use crate::mir::join_ir::lowering::JoinInlineBoundaryBuilder;
+        use crate::mir::join_ir::lowering::inline_boundary::LoopExitBinding;
+        use crate::mir::join_ir::lowering::carrier_info::CarrierRole;
+
+        // Phase 132-Post: Extract k_exit's parameter ValueId from join_module (Box-First)
+        let k_exit_func = join_module.require_function("k_exit", "Pattern 1");
+        let join_exit_value = k_exit_func.params.first().copied()
+            .expect("k_exit must have parameter for exit value");
+
+        // Phase 132: Create exit binding for loop variable
+        let exit_binding = LoopExitBinding {
+            carrier_name: ctx.loop_var_name.clone(),
+            join_exit_value,
+            host_slot: ctx.loop_var_id,
+            role: CarrierRole::LoopState,
+        };
+
         let boundary = JoinInlineBoundaryBuilder::new()
             .with_inputs(
                 vec![ValueId(0)],      // JoinIR's main() parameter (loop variable)
                 vec![ctx.loop_var_id], // Host's loop variable
             )
+            .with_exit_bindings(vec![exit_binding]) // Phase 132: Enable exit PHI & variable_map update
             .with_loop_var_name(Some(ctx.loop_var_name.clone())) // Phase 33-16: Enable header PHI generation for SSA correctness
             .build();