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/llvm_py/phi_manager.py

@@ -0,0 +1,47 @@
+"""
+Phase 132-Post: PHI Management Box
+
+Box-First principle: Encapsulate PHI lifecycle management
+- Track PHI ownership (which block created which PHI)
+- Protect PHIs from overwrites (SSOT principle)
+- Filter vmap to preserve PHI values
+"""
+
+class PhiManager:
+    """PHI value lifecycle manager (Box pattern)"""
+
+    def __init__(self):
+        self.predeclared = {}  # (bid, vid) -> phi_value
+
+    def register_phi(self, bid: int, vid: int, phi_value):
+        """Register a PHI as owned by specific block"""
+        self.predeclared[(bid, vid)] = phi_value
+
+    def is_phi_owned(self, bid: int, vid: int) -> bool:
+        """Check if PHI is owned by block"""
+        return (bid, vid) in self.predeclared
+
+    def filter_vmap_preserve_phis(self, vmap: dict, target_bid: int) -> dict:
+        """Filter vmap while preserving owned PHIs
+
+        SSOT: PHIs in vmap are the single source of truth
+        """
+        result = {}
+        for vid, val in vmap.items():
+            if hasattr(val, 'add_incoming'):  # Is PHI?
+                if self.is_phi_owned(target_bid, vid):
+                    result[vid] = val
+            else:
+                result[vid] = val
+        return result
+
+    def sync_protect_phis(self, target_vmap: dict, source_vmap: dict):
+        """Sync values but protect existing PHIs (Fail-Fast)
+
+        Never overwrite PHIs - they are SSOT
+        """
+        for vid, val in source_vmap.items():
+            existing = target_vmap.get(vid)
+            if existing and hasattr(existing, 'add_incoming'):
+                continue  # SSOT: Don't overwrite PHIs
+            target_vmap[vid] = val