feat(hako_check): Phase 154 MIR CFG integration & HC020 dead block detection

Implements block-level unreachable code detection using MIR CFG information.
Complements Phase 153's method-level HC019 with fine-grained analysis.

Core Infrastructure (Complete):
- CFG Extractor: Extract block reachability from MirModule
- DeadBlockAnalyzerBox: HC020 rule for unreachable blocks
- CLI Integration: --dead-blocks flag and rule execution
- Test Cases: 4 comprehensive patterns (early return, constant false, infinite loop, break)
- Smoke Test: Validation script for all test cases

Implementation Details:
- src/mir/cfg_extractor.rs: New module for CFG→JSON extraction
- tools/hako_check/rules/rule_dead_blocks.hako: HC020 analyzer box
- tools/hako_check/cli.hako: Added --dead-blocks flag and HC020 integration
- apps/tests/hako_check/test_dead_blocks_*.hako: 4 test cases

Architecture:
- Follows Phase 153 boxed modular pattern (DeadCodeAnalyzerBox)
- Optional CFG field in Analysis IR (backward compatible)
- Uses MIR's built-in reachability computation
- Gracefully skips if CFG unavailable

Known Limitation:
- CFG data bridge pending (Phase 155): analysis_consumer.hako needs MIR access
- Current: DeadBlockAnalyzerBox implemented, but CFG not yet in Analysis IR
- Estimated 2-3 hours to complete bridge in Phase 155

Test Coverage:
- Unit tests: cfg_extractor (simple CFG, unreachable blocks)
- Integration tests: 4 test cases ready (will activate with bridge)
- Smoke test: tools/hako_check_deadblocks_smoke.sh

Documentation:
- phase154_mir_cfg_inventory.md: CFG structure investigation
- phase154_implementation_summary.md: Complete implementation guide
- hako_check_design.md: HC020 rule documentation

Next Phase 155:
- Implement CFG data bridge (extract_mir_cfg builtin)
- Update analysis_consumer.hako to call bridge
- Activate HC020 end-to-end testing

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

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

docs/development/issues/llvm_binop_string_mismatch.md

@@ -1,6 +1,6 @@
 # LLVM lowering: string + int causes binop type mismatch
 
-Status: open
+Status: open (issue memo; see roadmap/CURRENT_TASK for up-to-date status)
 
 Summary
 
@@ -37,4 +37,3 @@ Plan
 CI
 
 - Keep `apps/ny-llvm-smoke` OFF by default. Re-enable once concat shim lands and binop lowering is updated.
-