feat(mir): Phase 136 Step 2/7 - CoreContext extraction complete

Extract core ID generation fields from MirBuilder to improve organization:
- value_gen: ValueIdGenerator
- block_gen: BasicBlockIdGenerator
- next_binding_id: u32 (Phase 74)
- temp_slot_counter: u32
- debug_join_counter: u32

Implementation:
- Created src/mir/builder/core_context.rs (215 lines)
- Added core_ctx field to MirBuilder as SSOT
- Deprecated legacy fields with backward compat
- ID allocation methods use core_ctx and sync legacy fields
- Added next_block_id() helper, replaced 30+ block_gen.next() calls

Testing:
- cargo build --release: SUCCESS (193 warnings expected)
- cargo test --release --lib: 1004/1004 PASS (+7 tests)
- phase135_trim_mir_verify.sh: PASS
- phase132_exit_phi_parity.sh: 3/3 PASS

Progress: 2/7 Context extractions complete (TypeContext + CoreContext)

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

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

src/mir/builder/control_flow/exception/try_catch.rs

@@ -32,14 +32,14 @@ pub(in crate::mir::builder) fn cf_try_catch(
         return Ok(result);
     }
 
-    let try_block = builder.block_gen.next();
-    let catch_block = builder.block_gen.next();
+    let try_block = builder.next_block_id();
+    let catch_block = builder.next_block_id();
     let finally_block = if finally_body.is_some() {
-        Some(builder.block_gen.next())
+        Some(builder.next_block_id())
     } else {
         None
     };
-    let exit_block = builder.block_gen.next();
+    let exit_block = builder.next_block_id();
 
     // Snapshot deferred-return state
     let saved_defer_active = builder.return_defer_active;