feat(phase-91): Phase 91 Step 2-B/2-D - P5b escape sequence pattern recognition
### Changes
#### 1. UpdateKind Extension (skeleton_types.rs)
- Added `ConditionalStep { then_delta: i64, else_delta: i64 }` variant
- Enables P5b patterns with conditional numeric deltas
- Used for escape sequence handling (i.e., +2 vs +1 based on escape char)
#### 2. Canonicalizer Integration (canonicalizer.rs)
- Added P5b pattern detection after existing patterns
- Routes to Pattern2Break (same as skip_whitespace, reflects has_break=true)
- Builds LoopSkeleton with ConditionalStep update
- Position: AFTER skip_whitespace (Pattern2Break refinement, not separate choice)
#### 3. AST Recognizer Enhancement (ast_feature_extractor.rs)
- Updated `find_escape_in_if` to handle both:
- `if ch == '\\' { i += 2 }` (no else)
- `if ch == '\\' { i += 2 } else { i += 1 }` (with else)
- Added `extract_delta_pair_from_if` for clean delta extraction
- Extracts counter_name, escape_delta, normal_delta from single if-else statement
#### 4. Comprehensive Unit Test (canonicalizer.rs)
- Test: `test_escape_skip_pattern_recognition`
- Verifies full P5b pattern recognition
- Confirms ConditionalStep with escape_delta=2, normal_delta=1
- Validates skeleton structure and exit contract (has_break=true)
### Results
✅ Build: 1062/1062 tests PASS (+1 new P5b test)
✅ No regressions
✅ P5b pattern now recognized and routed correctly
### Next Steps
- Step 2-E: Parity verification with strict mode
- Step 2-F: Documentation updates
Phase 91 P5b implementation on track!
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
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