refactor(mir): Remove CoreContext legacy fields (Phase 2-2/7)

完全移行→削除の安全順序（Option C）に従い、CoreContext の
deprecated フィールドと sync helpers を完全削除。

## Changes
- Migrated all access sites to core_ctx.* (SSOT allocators)
  - utils.rs: value_gen (3 sites), block_gen (1 site), temp_slot_counter (1 site)
  - loop_api.rs: block_gen (1 site)
  - phi_helpers.rs: value_gen (1 site)
  - builder.rs: sync helpers (2 methods), test assertions (2 sites)
- Removed 5 deprecated fields from builder.rs
  - value_gen: ValueIdGenerator
  - block_gen: BasicBlockIdGenerator
  - next_binding_id: u32
  - temp_slot_counter: u32
  - debug_join_counter: u32
- Removed sync helper calls (allocate_binding_id, debug_next_join_id)
- Removed field initializations from MirBuilder::new()
- Maintained Phase 136 P0 SSOT structure (next_value_id() remains as high-level API)

## Tests
- cargo build --release: PASS
- cargo test --release --lib: 1029 passed (4 pre-existing failures)
- Deprecation warnings: 435 → 456 (+21)
  Note: Warnings increased due to remaining deprecated field *uses* being exposed
  after removing the field *definitions*. This is expected during migration.

## Code metrics
- Net reduction: 40 lines (-56 removed, +16 modifications)
- Files changed: 4 (builder.rs, utils.rs, loop_api.rs, phi_helpers.rs)

Phase 2 Progress: 2/7 contexts complete (MetadataContext ✅, CoreContext ✅)

src/mir/builder/utils.rs

@@ -41,10 +41,8 @@ impl super::MirBuilder {
             let candidate = if let Some(ref mut f) = self.current_function {
                 f.next_value_id() // Function context
             } else {
-                // Phase 136 Step 2/7: Use core_ctx as SSOT, sync legacy field
-                let id = self.core_ctx.next_value();
-                self.value_gen = self.core_ctx.value_gen.clone();
-                id
+                // Phase 136 Step 2/7 + Phase 2-2: Use core_ctx as SSOT (no sync needed)
+                self.core_ctx.next_value()
             };
 
             // Phase 201-A: Skip reserved PHI dst ValueIds
@@ -60,13 +58,10 @@ impl super::MirBuilder {
 
     /// Allocate a new BasicBlockId
     ///
-    /// Phase 136 Step 2/7: Uses core_ctx as SSOT, syncs legacy field.
+    /// Phase 136 Step 2/7 + Phase 2-2: Uses core_ctx as SSOT (no sync needed).
     #[inline]
-    #[allow(deprecated)]
     pub(crate) fn next_block_id(&mut self) -> super::BasicBlockId {
-        let id = self.core_ctx.next_block();
-        self.block_gen = self.core_ctx.block_gen.clone();
-        id
+        self.core_ctx.next_block()
     }
 
     // ---- LocalSSA convenience (readability helpers) ----
@@ -442,13 +437,13 @@ impl super::MirBuilder {
         v: super::ValueId,
         hint: &str,
     ) -> Result<super::ValueId, String> {
-        self.temp_slot_counter = self.temp_slot_counter.wrapping_add(1);
-        let slot_name = format!("__pin${}${}", self.temp_slot_counter, hint);
+        let slot_id = self.core_ctx.next_temp_slot();
+        let slot_name = format!("__pin${}${}", slot_id, hint);
         // Phase 25.1b: Use function-local ID allocator to avoid SSA verification failures
         let dst = if let Some(ref mut f) = self.current_function {
             f.next_value_id() // Function context: use local ID
         } else {
-            self.value_gen.next() // Module context: use global ID
+            self.core_ctx.next_value() // Module context: use core_ctx SSOT
         };
         self.emit_instruction(super::MirInstruction::Copy { dst, src: v })?;
         if super::utils::builder_debug_enabled()
@@ -479,7 +474,7 @@ impl super::MirBuilder {
         let dst = if let Some(ref mut f) = self.current_function {
             f.next_value_id() // Function context: use local ID
         } else {
-            self.value_gen.next() // Module context: use global ID
+            self.core_ctx.next_value() // Module context: use core_ctx SSOT
         };
         self.emit_instruction(super::MirInstruction::Copy { dst, src: v })?;
         // Propagate metadata (type/origin) from source to the new local copy