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nyash-codex 9e3b258046 feat(phase-91): JoinIR Selfhost depth-2 advancement - Pattern P5b design & planning

## Overview
Analyzed 34 loops across selfhost codebase to identify JoinIR coverage gaps.
Current readiness: 47% (16/30 loops). Next frontier: Pattern P5b (Escape Handling).

## Current Status
- Phase 91 planning document: Complete
  - Loop inventory across 6 key files
  - Priority ranking: P5b (escape) > P5 (guard) > P6 (nested)
  - Effort estimates and ROI analysis

- Pattern P5b Design: Complete
  - Problem statement (variable-step carriers)
  - Pattern definition with Skeleton layout
  - Recognition algorithm (8-step detection)
  - Capability taxonomy (P5b-specific guards)
  - Lowering strategy (Phase 92 preview)

- Test fixture: Created
  - Minimal escape sequence parser
  - JSON string with backslash escape

- Loop Canonicalizer extended
  - Capability table updated with P5b entries
  - Fail-Fast criteria documented
  - Implementation checklist added

## Key Findings

### Loop Readiness Matrix
| Category | Count | JoinIR Status |
|----------|-------|--------------|
| Pattern 1 (simple bounded) | 16 | ✅ Ready |
| Pattern 2 (with break) | 1 | ⚠️ Partial |
| **Pattern P5b (escape seq)** | ~3 | ❌ NEW |
| Pattern P5 (guard-bounded) | ~2 | ❌ Deferred |
| Pattern P6 (nested loops) | ~8 | ❌ Deferred |

### Top Candidates
1. **P5b**: json_loader.hako:30 (8 lines, high reuse)
   - Effort: 2-3 days (recognition)
   - Impact: Unlocks all escape parsers

2. **P5**: mini_vm_core.hako:541 (204 lines, monolithic)
   - Effort: 1-2 weeks
   - Impact: Major JSON optimization

3. **P6**: seam_inspector.hako:76 (7+ nesting)
   - Effort: 2-3 weeks
   - Impact: Demonstrates nested composition

## Phase 91 Strategy
**Recognition-only phase** (no lowering in P1):
- Step 1: Design & planning ✅
- Step 2: Canonicalizer implementation (detect_escape_pattern)
- Step 3: Unit tests + parity verification
- Step 4: Lowering deferred to Phase 92

## Files Added
- docs/development/current/main/phases/phase-91/README.md - Full analysis & planning
- docs/development/current/main/design/pattern-p5b-escape-design.md - Technical design
- tools/selfhost/test_pattern5b_escape_minimal.hako - Test fixture

## Files Modified
- docs/development/current/main/design/loop-canonicalizer.md
  - Capability table extended with P5b entries
  - Pattern P5b full section added
  - Implementation checklist updated

## Acceptance Criteria (Phase 91 Step 1)
- ✅ Loop inventory complete (34 loops across 6 files)
- ✅ Pattern P5b design document ready
- ✅ Test fixture created
- ✅ Capability taxonomy extended
- ⏳ Implementation deferred (Step 2+)

## References
- JoinIR Architecture: joinir-architecture-overview.md
- Phase 91 Plan: phases/phase-91/README.md
- P5b Design: design/pattern-p5b-escape-design.md

Next: Implement detect_escape_pattern() recognition in Phase 91 Step 2

🤖 Generated with Claude Code

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