fix(parse-pdf): strip Identity-H Unimplemented marker + dominance heuristic in compute_valid_char_ratio (Bug #6)

Why: metro-korea.pdf (Identity-H CID font without ToUnicode CMap) 의
ingest 가 pdf_ocr_pages=0 으로 잘못 종료. lopdf 0.32.0 의 emit
`?Identity-H Unimplemented?` marker 28 ASCII char 가 is_valid_text_char()
의 0x0020..=0x007E range 통과 → ratio=1.0 → OCR fallback 0.5
threshold bypass.

Change: MOJIBAKE_MARKERS const + compute_valid_char_ratio() 4-단계
(strip → trim-empty zero → dominance cap-0.3 → 기존 ratio). marker
list extensible. is_valid_text_char() 본체 변경 0.

Tests: +2 unit (dominance + minority) on top of 기존 8. parser_version
/ wire schema 변경 0.

Refs: docs/superpowers/specs/2026-05-27-v0.20-sub1-bugfix2-spec.md
§4.1 / §4.2 / §6 R-1.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tasks/HOTFIXES.md

@@ -14,6 +14,13 @@ historical contract that was implemented; this file accumulates the
 deltas so phase 5+ readers can find the live behavior without diffing
 git history.
 
+## 2026-05-27 — Identity-H mojibake marker bypassed OCR fallback (Bug #6)
+
+- **Symptom**: `metro-korea.pdf` (Identity-H CID font without ToUnicode CMap) 의 ingest 가 `pdf_ocr_pages=0` 으로 종료. text 전체가 `?Identity-H Unimplemented?` marker 1154회 반복 (lopdf 0.32.0 emit). text-detect ratio = 1.0 → OCR fallback threshold 0.5 bypass.
+- **Root cause**: `crates/kebab-parse-pdf/src/text_quality.rs::compute_valid_char_ratio()` 의 `is_valid_text_char()` 가 ASCII printable range (0x0020..=0x007E) 를 unconditional valid 처리. marker (28 ASCII char) 는 valid 로 count.
+- **Fix**: `MOJIBAKE_MARKERS` const 도입 + marker strip after-strip 의 trim-empty → 0.0 + dominance heuristic (strip > 잔여 일 때 cap 0.3). spec ACCEPT: `docs/superpowers/specs/2026-05-27-v0.20-sub1-bugfix2-spec.md` §4.1. parser_version/wire schema 영향 0.
+- **User action**: 이미 `metro-korea.pdf` class 의 mojibake-heavy PDF 를 v0.20.0 pre-bugfix2 binary 로 indexed 한 경우, `kebab ingest --force-reingest <workspace>` 로 cached skip 무효화 필요 (release notes 동등 안내).
+
 ## 2026-05-27 — v0.20.0 sub-item 1: chunk_id `#c{char_start}` workaround collapses under aggressive overlap (Bug #3 second-iteration patch)
 
 **Symptom**: F2 (1580 chars OCR, scanned_page2.pdf) ingest 시