refactor(llvm): Phase 132-P1 - FunctionLowerContext Box isolation

Structural fix for cross-function state leakage bugs discovered in Phase 131-132.

Problem:
- Function-local state (block_end_values, def_blocks, phi_manager, caches)
  was globally managed, causing collisions across functions
- Required manual clearing via _clear_function_local_state() (~80 lines)
- Tuple-key workaround (func_name, block_id) added complexity

Solution: FunctionLowerContext Box
- Encapsulates all function-scoped state in a dedicated Box
- Automatic lifetime management (created at entry, destroyed at exit)
- Eliminates manual state clearing
- Simplifies from tuple-key to simple block_id access

Implementation:
1. Created src/llvm_py/context/function_lower_context.py (150+ lines)
   - block_end_values, def_blocks, jump_only_blocks
   - phi_manager, resolver_caches
   - Helper methods: get/set_block_snapshot(), register_def(), etc.

2. Updated function_lower.py
   - Creates context at function entry
   - Binds resolver to context for cache isolation
   - Removed _clear_function_local_state() (~80 lines)

3. Updated block_lower.py
   - Changed tuple-key (func_name, block_id) to simple block_id
   - Access via context.get_block_snapshot() / context.set_block_snapshot()

4. Updated resolver.py, phi_wiring/wiring.py, phi_wiring/tagging.py
   - All state access now through context

5. Fixed critical bug in phi_wiring/tagging.py
   - setup_phi_placeholders() was breaking context connection
   - Changed reassignment to .clear()/.update() to preserve reference

Benefits:
- ✅ Automatic state isolation (no manual clearing)
- ✅ Clear ownership (one context per function)
- ✅ Eliminated tuple-keys (simpler code)
- ✅ Prevents bugs by design (cross-function leakage impossible)

Test results:
- ✅ Phase 87 smoke test: PASS
- ✅ Phase 132 smoke test: PASS (both cases)
- ✅ STRICT mode: Works with multi-function inputs
- ✅ No regression

Files modified: 8 (2 new, 6 updated)

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

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

src/llvm_py/llvm_builder.py

@@ -112,8 +112,9 @@ class NyashLLVMBuilder:
         self.phi_deferrals: List[Tuple[int, int, List[Tuple[int, int]]]] = []
         # Predecessor map and per-block end snapshots
         self.preds: Dict[int, List[int]] = {}
-        # Phase 132-P0: Tuple-key (func_name, block_id) to prevent cross-function collision
-        self.block_end_values: Dict[Tuple[str, int], Dict[int, ir.Value]] = {}
+        # Phase 132-P1: Legacy storage (replaced by FunctionLowerContext Box per-function)
+        # These are now only used as fallback/backward compatibility
+        self.block_end_values: Dict[int, Dict[int, ir.Value]] = {}
         # Definition map: value_id -> set(block_id) where the value is defined
         # Used as a lightweight lifetime hint to avoid over-localization
         self.def_blocks: Dict[int, set] = {}