feat(joinir): Phase 240-EX - Pattern2 header condition ExprLowerer integration

Implementation:
- Add make_pattern2_scope_manager() helper for DRY
- Header conditions use ExprLowerer for supported patterns
- Legacy fallback for unsupported patterns
- Fail-Fast on supported patterns that fail

Tests:
- 4 new tests (all pass)
- test_expr_lowerer_supports_simple_header_condition_i_less_literal
- test_expr_lowerer_supports_header_condition_var_less_var
- test_expr_lowerer_header_condition_generates_expected_instructions
- test_pattern2_header_condition_via_exprlowerer

Also: Archive old phase documentation (34k lines removed)

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

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

docs/development/testing/selfhost_exe_stageb_quick_guide.md

@@ -1,3 +1,7 @@
+Status: Active  
+Scope: `tools/selfhost_exe_stageb.sh` の具体的な使い方と代表パターン（EXE ビルド/VM とのパリティ検証）をまとめたガイド。  
+See also: `docs/development/selfhosting/quickstart.md`（Self‑Host 全体フローの SSOT）。
+
 # selfhost_exe_stageb.sh - Quick Usage Guide
 
 ## TL;DR