docs(pdftract-5qj50): add verification note

Closes: pdftract-5qj50

notes/pdftract-5qj50.md

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+# pdftract-5qj50: Mojibake detection + Latin-1-as-UTF-8 re-decode via encoding_rs
+
+## Summary
+
+Implemented `detect_and_repair_mojibake` function in Phase 4.7 Correction Pipeline.
+The function detects Latin-1 bytes misinterpreted as UTF-8 (e.g., "café" → "café")
+and attempts recovery via encoding_rs::WINDOWS_1252 re-decoding.
+
+## Implementation
+
+### Files Created
+- `crates/pdftract-core/src/layout/correction.rs` - New module with:
+  - `CorrectableText` trait for mutable text access
+  - `detect_and_repair_mojibake<T, F>(span, scorer) -> bool` function
+  - `TestCorrectable` for unit tests
+  - `contains_mojibake_indicators` helper function
+
+### Files Modified
+- `crates/pdftract-core/Cargo.toml` - Made encoding_rs non-optional (was cjk-gated)
+- `crates/pdftract-core/src/hybrid.rs` - Added CorrectableText impl for Span
+- `crates/pdftract-core/src/schema/mod.rs` - Added CorrectableText impl for SpanJson
+- `crates/pdftract-core/src/layout/mod.rs` - Added correction module and export
+
+### Key Features
+1. **Detection Heuristic**: Checks for ≥2 occurrences of telltale 2-char sequences:
+   - Latin-1 vowels: é, è, ê, î, ô, û, â, ç, ñ, etc.
+   - Windows-1252 smart quotes: ’, â€", “, â€
+   - NBSP pattern: Â followed by non-ASCII
+
+2. **Correction Process**:
+   - Encode text as UTF-8 bytes
+   - Decode bytes as windows-1252 via encoding_rs
+   - Score both original and candidate via scorer callback
+   - Accept if `candidate_score > original_score + 0.05`
+
+3. **Invariants**:
+   - Clean ASCII/pure UTF-8 pass-through unchanged (fast-path)
+   - Re-decoding REVERTED if score doesn't improve (false-positive safety)
+   - Uses windows-1252 (not pure Latin-1) for Microsoft-isms support
+
+## Acceptance Criteria Status
+
+| Criterion | Status | Notes |
+|-----------|--------|-------|
+| Span text "café" → no à detected → no change | PASS | Clean UTF-8 fast-path works |
+| Span text "café" → detected, re-decoded to "café" | PASS | Test: test_mojibake_detected_and_repaired |
+| Span text "cafésandbar" (one Ã) → threshold check | PASS | Below 2-indicator threshold, no change |
+| Span text "éèî" (multiple) → re-decoded if score improves | PASS | Test: test_mojibake_multiple_indicators |
+| Asian text → pass-through unaffected | PASS | Test: test_asian_text_unaffected |
+| Epsilon threshold (0.05) prevents noise | PASS | Tests: test_epsilon_threshold_prevents_noise, test_exact_epsilon_boundary |
+| Re-decoding rejected if score doesn't improve | PASS | Test: test_replacement_rejected_if_score_doesnt_improve |
+
+## Test Results
+
+All unit tests in `layout::correction::tests` pass:
+- test_clean_utf8_no_change ✓
+- test_ascii_only_no_change ✓
+- test_empty_string_no_change ✓
+- test_mojibake_detected_and_repaired ✓
+- test_mojibake_multiple_indicators ✓
+- test_mojibake_single_indicator_threshold ✓
+- test_smart_quote_mojibake ✓
+- test_em_dash_mojibake ✓
+- test_replacement_rejected_if_score_doesnt_improve ✓
+- test_epsilon_threshold_prevents_noise ✓
+- test_asian_text_unaffected ✓
+- test_windows1252_specific ✓
+- test_mixed_ascii_and_mojibake ✓
+- test_nbsp_indicator ✓
+- test_multiple_mojibake_patterns ✓
+- test_exact_epsilon_boundary ✓
+- test_just_above_epsilon ✓
+
+## Git Commits
+
+- `d84f8da` - feat(pdftract-5qj50): implement mojibake detection and repair via encoding_rs
+
+## References
+
+- Plan section: Phase 4.7 Correction pipeline step 3 (line 1797)
+- Critical test: "Latin-1 mojibake é corrected to é when re-decode raises readability score" (line 1792)