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/join_ir/lowering/simple_while_minimal.rs

@@ -25,13 +25,13 @@
 //!
 //! fn loop_step(i):
 //!   exit_cond = !(i < 3)
-//!   Jump(k_exit, [], cond=exit_cond)  // early return if i >= 3
-//!   print(i)                          // body
-//!   i_next = i + 1                    // increment
-//!   Call(loop_step, [i_next])         // tail recursion
+//!   Jump(k_exit, [i], cond=exit_cond)  // Phase 132: pass i to k_exit
+//!   print(i)                           // body
+//!   i_next = i + 1                     // increment
+//!   Call(loop_step, [i_next])          // tail recursion
 //!
-//! fn k_exit():
-//!   return 0
+//! fn k_exit(i_exit):                   // Phase 132: receives loop variable
+//!   return i_exit                      // Phase 132: return loop value
 //! ```
 //!
 //! ## Design Notes
@@ -118,7 +118,8 @@ pub(crate) fn lower_simple_while_minimal(
     let i_next = alloc_value(); // ValueId(8) - i + 1
 
     // k_exit locals
-    let const_0_exit = alloc_value(); // ValueId(9) - exit return value
+    // Phase 132: i_exit receives loop variable from Jump
+    let i_exit = alloc_value(); // ValueId(9) - exit parameter (loop variable)
 
     // ==================================================================
     // main() function
@@ -181,10 +182,11 @@ pub(crate) fn lower_simple_while_minimal(
             operand: cmp_lt,
         }));
 
-    // Jump(k_exit, [], cond=exit_cond)
+    // Phase 132: Jump(k_exit, [i_param], cond=exit_cond)
+    // Pass loop variable to exit continuation for return value parity
     loop_step_func.body.push(JoinInst::Jump {
         cont: k_exit_id.as_cont(),
-        args: vec![],
+        args: vec![i_param],
         cond: Some(exit_cond),
     });
 
@@ -224,19 +226,14 @@ pub(crate) fn lower_simple_while_minimal(
     join_module.add_function(loop_step_func);
 
     // ==================================================================
-    // k_exit() function
+    // k_exit(i_exit) function - Phase 132: receives loop variable
     // ==================================================================
-    let mut k_exit_func = JoinFunction::new(k_exit_id, "k_exit".to_string(), vec![]);
-
-    // return 0 (Pattern 1 has no exit values)
-    // Phase 188-Impl-3: Use pre-allocated const_0_exit (ValueId(9))
-    k_exit_func.body.push(JoinInst::Compute(MirLikeInst::Const {
-        dst: const_0_exit,
-        value: ConstValue::Integer(0),
-    }));
+    let mut k_exit_func = JoinFunction::new(k_exit_id, "k_exit".to_string(), vec![i_exit]);
 
+    // Phase 132: return i_exit (loop variable at exit)
+    // This ensures VM/LLVM parity for `return i` after loop
     k_exit_func.body.push(JoinInst::Ret {
-        value: Some(const_0_exit),
+        value: Some(i_exit),
     });
 
     join_module.add_function(k_exit_func);

src/mir/join_ir/mod.rs

@@ -585,6 +585,18 @@ impl JoinModule {
     pub fn mark_normalized(&mut self) {
         self.phase = JoinIrPhase::Normalized;
     }
+
+    // Phase 132-Post: Box-First principle - encapsulate function search logic
+    /// Find function by name
+    pub fn get_function_by_name(&self, name: &str) -> Option<&JoinFunction> {
+        self.functions.values().find(|f| f.name == name)
+    }
+
+    /// Find function by name or panic with descriptive message
+    pub fn require_function(&self, name: &str, context: &str) -> &JoinFunction {
+        self.get_function_by_name(name)
+            .unwrap_or_else(|| panic!("{}: missing required function '{}'", context, name))
+    }
 }
 
 impl Default for JoinModule {