refactor(joinir): Phase 85 - Quick wins: loop_patterns removal, DebugOutputBox, dead_code audit

Quick Win 1: Remove loop_patterns_old.rs (COMPLETED)
- Deleted obsolete legacy loop pattern dispatcher (914 lines)
- All patterns (Break/Continue/Simple) now in modular loop_patterns/ system
- Moved helper functions (has_break_in_loop_body, has_continue_in_loop_body) to analysis.rs
- Updated loop_frontend_binding.rs to remove fallback
- Verified zero regressions: 974/974 lib tests PASS

Quick Win 2: DebugOutputBox consolidation (COMPLETED)
- New module: src/mir/join_ir/lowering/debug_output_box.rs (170 lines)
- Centralized debug output management with automatic HAKO_JOINIR_DEBUG checking
- Refactored 4 files to use DebugOutputBox:
  - condition_env.rs: 3 scattered checks → 3 Box calls
  - carrier_binding_assigner.rs: 1 check → 1 Box call
  - scope_manager.rs: 3 checks → 3 Box calls
  - analysis.rs: Updated lower_loop_with_if_meta to use new pattern system
- Benefits: Consistent formatting, centralized control, zero runtime cost when disabled
- Added 4 unit tests for DebugOutputBox

Quick Win 3: Dead code directive audit (COMPLETED)
- Audited all 40 #[allow(dead_code)] directives in lowering/
- Findings: All legitimate (Phase utilities, future placeholders, API completeness)
- No unsafe removals needed
- Categories:
  - Phase 192 utilities (whitespace_check, entry_builder): Public API with tests
  - Phase 231 placeholders (expr_lowerer): Explicitly marked future use
  - Const helpers (value_id_ranges): API completeness
  - Loop metadata (loop_update_summary): Future phase fields

Result: Net -858 lines, improved code clarity, zero regressions
Tests: 974/974 PASS (gained 4 from DebugOutputBox tests)

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

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

src/mir/join_ir/frontend/ast_lowerer/mod.rs

@@ -32,7 +32,7 @@ mod if_in_loop;
 mod if_return;
 mod loop_frontend_binding;
 mod loop_patterns;
-mod loop_patterns_old;
+// Removed: loop_patterns_old (obsolete legacy dispatcher, all patterns now in loop_patterns/)
 mod nested_if;
 mod read_quoted;
 mod stmt_handlers;