feat(mir): Phase 143 P1 - Add parse_string pattern to canonicalizer
Expand loop canonicalizer to recognize parse_string patterns with both
continue (escape handling) and return (quote found) statements.
## Implementation
### New Pattern Detection (ast_feature_extractor.rs)
- Add `detect_parse_string_pattern()` function
- Support nested continue detection using `has_continue_node()` helper
- Recognize both return and continue in same loop body
- Return ParseStringInfo { carrier_name, delta, body_stmts }
- ~120 lines added
### Canonicalizer Integration (canonicalizer.rs)
- Try parse_string pattern first (most specific)
- Build LoopSkeleton with HeaderCond, Body, Update steps
- Set ExitContract: has_continue=true, has_return=true
- Route to Pattern4Continue (both exits present)
- ~45 lines modified
### Export Chain
- Add re-exports through 7 module levels:
ast_feature_extractor → patterns → joinir → control_flow → builder → mir
- 10 lines total across 7 files
### Unit Test
- Add `test_parse_string_pattern_recognized()` in canonicalizer.rs
- Verify skeleton structure (3+ steps)
- Verify carrier (name="p", delta=1, role=Counter)
- Verify exit contract (continue=true, return=true, break=false)
- Verify routing decision (Pattern4Continue, no missing_caps)
- ~180 lines added
## Target Pattern
`tools/selfhost/test_pattern4_parse_string.hako`
Pattern structure:
- Check for closing quote → return
- Check for escape sequence → continue (nested inside another if)
- Regular character processing → p++
## Results
- ✅ Strict parity green: Pattern4Continue
- ✅ All 19 unit tests pass
- ✅ Nested continue detection working
- ✅ ExitContract correctly set (first pattern with both continue+return)
- ✅ Default behavior unchanged
## Technical Challenges
1. Nested continue detection required recursive search
2. First pattern with both has_continue=true AND has_return=true
3. Variable step updates (p++ vs p+=2) handled with base delta
## Statistics
- New patterns: 1 (parse_string)
- Total patterns: 4 (skip_whitespace, parse_number, continue, parse_string)
- New capabilities: 0 (uses existing ConstStep)
- Lines added: ~300
- Files modified: 9
- Parity status: Green ✅
Phase 143 P1: Complete
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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