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)

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

docs/development/selfhosting/quickstart.md

@@ -1,3 +1,5 @@
+Status: SSOT, Active
+
 # Self‑Hosting Quickstart (Phase 15 — Resume)
 
 This note shows how to run the Nyash self‑host compiler MVP to emit MIR(JSON v0) and execute it with the current VM line. The flow keeps defaults unchanged and uses small, opt‑in flags for development.