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/loop_api.rs

@@ -83,7 +83,7 @@ pub fn build_simple_loop<L: LoopBuilderApi>(
 // === Legacy wiring: implement LoopBuilderApi for mir::builder::MirBuilder ===
 impl LoopBuilderApi for super::builder::MirBuilder {
     fn new_block(&mut self) -> BasicBlockId {
-        self.block_gen.next()
+        self.core_ctx.next_block()
     }
     fn current_block(&self) -> Result<BasicBlockId, String> {
         self.current_block