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>

src/mir/mod.rs

@@ -31,6 +31,7 @@ pub mod join_ir_ops; // Phase 27.8: JoinIR 命令意味箱（ops box）
 pub mod join_ir_runner; // Phase 27.2: JoinIR 実行器（実験用）
 pub mod join_ir_vm_bridge; // Phase 27-shortterm S-4: JoinIR → Rust VM ブリッジ
 pub mod join_ir_vm_bridge_dispatch; // Phase 30 F-4.4: JoinIR VM ブリッジ dispatch helper
+pub mod cfg_extractor; // Phase 154: CFG extraction for hako_check
 pub mod loop_form; // ControlForm::LoopShape の薄いエイリアス
 pub mod optimizer_passes; // optimizer passes (normalize/diagnostics)
 pub mod optimizer_stats; // extracted stats struct
@@ -50,6 +51,7 @@ pub mod verification_types; // extracted error types // Optimization subpasses (
 // Re-export main types for easy access
 pub use basic_block::{BasicBlock, BasicBlockId, BasicBlockIdGenerator};
 pub use builder::MirBuilder;
+pub use cfg_extractor::extract_cfg_info; // Phase 154: CFG extraction
 pub use definitions::{CallFlags, Callee, MirCall}; // Unified call definitions
 pub use effect::{Effect, EffectMask};
 pub use function::{FunctionSignature, MirFunction, MirModule};