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/joinir/merge/block_allocator.rs

@@ -23,7 +23,7 @@ pub(super) fn allocate_blocks(
 
     // Phase 177-3: Allocate exit block FIRST to ensure it doesn't conflict with JoinIR blocks
     // This exit_block_id will be returned and used by instruction_rewriter and exit_phi_builder
-    let exit_block_id = builder.block_gen.next();
+    let exit_block_id = builder.next_block_id();
 
     eprintln!(
         "[cf_loop/joinir/block_allocator] Phase 177-3: Allocated exit_block_id = {:?}",
@@ -49,7 +49,7 @@ pub(super) fn allocate_blocks(
         blocks.sort_by_key(|(id, _)| id.0);
 
         for (old_block_id, _) in blocks {
-            let new_block_id = builder.block_gen.next();
+            let new_block_id = builder.next_block_id();
             // Use remapper to store composite key mapping
             remapper.set_block(func_name.clone(), *old_block_id, new_block_id);