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/exprs_qmark.rs

@@ -18,8 +18,8 @@ impl super::MirBuilder {
             method_id: None,
             effects: super::EffectMask::PURE,
         })?;
-        let then_block = self.block_gen.next();
-        let else_block = self.block_gen.next();
+        let then_block = self.next_block_id();
+        let else_block = self.next_block_id();
         let ok_local = self.local_ssa_ensure(ok_id, 4);
         crate::mir::builder::emission::branch::emit_conditional(
             self, ok_local, then_block, else_block,