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>

src/mir/loop_canonicalizer/pattern_recognizer.rs

@@ -6,6 +6,7 @@
 use crate::ast::ASTNode;
 use crate::mir::detect_continue_pattern;
 use crate::mir::detect_parse_number_pattern as ast_detect_parse_number;
+use crate::mir::detect_parse_string_pattern as ast_detect_parse_string;
 use crate::mir::detect_skip_whitespace_pattern as ast_detect;
 
 // ============================================================================
@@ -75,6 +76,39 @@ pub fn try_extract_parse_number_pattern(
     })
 }
 
+// ============================================================================
+// Parse String Pattern (Phase 143-P1)
+// ============================================================================
+
+/// Try to extract parse_string pattern from loop
+///
+/// Pattern structure:
+/// ```
+/// loop(cond) {
+///     // ... body statements (ch computation)
+///     if quote_cond {
+///         return result
+///     }
+///     if escape_cond {
+///         // ... escape handling
+///         carrier = carrier + const
+///         continue
+///     }
+///     // ... regular character handling
+///     carrier = carrier + const
+/// }
+/// ```
+///
+/// Returns (carrier_name, delta, body_stmts) if pattern matches.
+///
+/// # Phase 143-P1: Parse String Pattern Detection
+///
+/// This function delegates to `ast_feature_extractor::detect_parse_string_pattern`
+/// for SSOT implementation.
+pub fn try_extract_parse_string_pattern(body: &[ASTNode]) -> Option<(String, i64, Vec<ASTNode>)> {
+    ast_detect_parse_string(body).map(|info| (info.carrier_name, info.delta, info.body_stmts))
+}
+
 // ============================================================================
 // Continue Pattern (Phase 142-P1)
 // ============================================================================