feat(mir): Phase 136 P0 - ValueId allocator SSOT 徹底（関数内経路から value_gen.next() 掃討）

## Summary
Eliminates remaining `value_gen.next()` calls from function-context code paths,
unifying all ValueId allocation through `MirBuilder::next_value_id()` SSOT allocator.

## Changes

### 1. Fixed `new_typed_value()` (src/mir/builder.rs:1068)
**Before**: `let id = self.value_gen.next();` (bypasses function context)
**After**: `let id = self.next_value_id();` (respects function context)

This is a public API used in function context, so must use SSOT allocator
to avoid collisions with reserved PHI dsts and function params.

### 2. Fixed test code (src/mir/builder.rs)
**test_shadowing_binding_restore** (lines 1161, 1171):
- Simulates function scope with `push_lexical_scope()`
- Changed to `builder.next_value_id()` for function scope simulation

**test_valueid_binding_parallel_allocation** (lines 1196-1216):
- Tests ValueId/BindingId independence
- Changed to `builder.next_value_id()` with note that Module context fallback preserves test intent

### 3. Verified Module context fallbacks (OK, no change needed)
These already check `current_function.is_some()` and use `value_gen.next()` only as Module context fallback:
- `src/mir/builder/utils.rs:43` - next_value_id() SSOT implementation
- `src/mir/builder/utils.rs:436` - pin_to_slot()
- `src/mir/builder/utils.rs:467` - materialize_local()
- `src/mir/utils/phi_helpers.rs:69` - insert_phi_unified()

## Verification
```bash
rg -n "value_gen\.next\(" src/mir --type rust | grep -v "Module context" | grep -v "//"
# Result: Only comments/docs remain
```

## Acceptance
✅ cargo test --release --lib - 997 passed
✅ phase135_trim_mir_verify.sh - PASS
✅ phase132_exit_phi_parity.sh - 3/3 PASS
✅ All function-context `value_gen.next()` eliminated

## Effect
- **Collision prevention**: No more ValueId collisions between function-level allocations and reserved PHI dsts
- **SSOT compliance**: All ValueId allocation flows through single allocator
- **Contract enforcement**: Phase 135 P1 contract_checks will catch violations immediately

🤖 Generated with Claude Code

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

src/mir/builder.rs

@@ -1064,8 +1064,9 @@ impl MirBuilder {
     // ============================================================================
 
     /// 型付きValueIdを発行（新API）
+    /// Phase 136 P0: Use SSOT allocator (next_value_id) to respect function context
     pub fn new_typed_value(&mut self, kind: super::MirValueKind) -> super::TypedValueId {
-        let id = self.value_gen.next();
+        let id = self.next_value_id();
         self.value_kinds.insert(id, kind);
         super::TypedValueId::new(id, kind)
     }
@@ -1156,7 +1157,8 @@ mod binding_id_tests {
         builder.push_lexical_scope();
 
         // Declare outer x
-        let outer_vid = builder.value_gen.next();
+        // Phase 136 P0: Use SSOT allocator for function scope simulation
+        let outer_vid = builder.next_value_id();
         builder
             .declare_local_in_current_scope("x", outer_vid)
             .unwrap();
@@ -1165,7 +1167,8 @@ mod binding_id_tests {
 
         // Enter inner scope and shadow x
         builder.push_lexical_scope();
-        let inner_vid = builder.value_gen.next();
+        // Phase 136 P0: Use SSOT allocator for function scope simulation
+        let inner_vid = builder.next_value_id();
         builder
             .declare_local_in_current_scope("x", inner_vid)
             .unwrap();
@@ -1186,10 +1189,13 @@ mod binding_id_tests {
     fn test_valueid_binding_parallel_allocation() {
         let mut builder = MirBuilder::new();
 
+        // Phase 136 P0: Use SSOT allocator (next_value_id)
+        // Note: Without current_function, next_value_id() falls back to value_gen.next()
+        // so this test still validates ValueId/BindingId independence
         // Allocate ValueIds and BindingIds in parallel
-        let vid0 = builder.value_gen.next();
+        let vid0 = builder.next_value_id();
         let bid0 = builder.allocate_binding_id();
-        let vid1 = builder.value_gen.next();
+        let vid1 = builder.next_value_id();
         let bid1 = builder.allocate_binding_id();
 
         // ValueId and BindingId should be independent
@@ -1199,15 +1205,15 @@ mod binding_id_tests {
         assert_eq!(bid1.raw(), 1);
 
         // Allocating more ValueIds should not affect BindingId counter
-        let _ = builder.value_gen.next();
-        let _ = builder.value_gen.next();
+        let _ = builder.next_value_id();
+        let _ = builder.next_value_id();
         let bid2 = builder.allocate_binding_id();
         assert_eq!(bid2.raw(), 2); // Still sequential
 
         // Allocating more BindingIds should not affect ValueId counter
         let _ = builder.allocate_binding_id();
         let _ = builder.allocate_binding_id();
-        let vid2 = builder.value_gen.next();
+        let vid2 = builder.next_value_id();
         assert_eq!(vid2.0, 4); // Continues from where we left off
     }
 }