fix(parse-pdf): F4 mojibake.pdf via pikepdf surgery; preserve 1-page invariant (Bug #4)

v0.20.0 sub-item 1 dogfood report 의 Bug #4 — F4 mojibake.pdf 의 lopdf
`get_pages()` count = 0 (Pages tree broken). root cause = 기존 byte-
level `re.sub` + manual startxref edit 가 lopdf strict load 통과시키지만
Pages dict 의 `/Kids` reference 깨짐.

- `tests/fixtures/_synth/mojibake.py`: full rewrite — replace byte-level
  `re.sub` + manual startxref with pikepdf open+inject-dummy-ToUnicode+
  del+save (auto xref regen). HYSMyeongJo-Medium CID font: CID font 이
  ToUnicode 를 자체 생성하지 않아 dummy stream 을 inject 후 strip
  (removed=1 invariant). Exit codes 2/3/4 for invariant fail.
- `crates/kebab-parse-pdf/tests/fixtures/mojibake.pdf`: regenerate via
  pikepdf — 1 valid page, no /ToUnicode marker, byte-identical 후 reproducible.
- `crates/kebab-parse-pdf/tests/snapshots/vector_pdf_canonical.json`:
  regen via 2-run cargo test pattern (hand-rolled unwrap_or_else baseline
  bootstrap, no insta crate).
- `crates/kebab-parse-pdf/tests/text_extractor_regression.rs`: append 3
  invariant test — (1) lopdf 1-page, (2) /ToUnicode marker absent,
  (3) PdfTextExtractor 1-block invariant.
- `crates/kebab-parse-pdf/src/text_quality.rs`: f4_fixture_ratio_under_threshold
  threshold 0.3 → 0.5 (production valid_ratio_threshold 기본값). 구 broken
  fixture (pages=0) 는 extract_text="" → ratio=0.0; 신 fixed fixture 는
  CID 2-byte fallback decode → ratio≈0.375 — 여전히 OCR trigger 조건 충족.

spec: docs/superpowers/specs/2026-05-27-v0.20-sub1-bugfix-spec.md (§5)
plan: docs/superpowers/plans/2026-05-27-v0.20-sub1-bugfix-plan.md (Step 4)
prior: 241ded5 (Step 3 integration test)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/kebab-parse-pdf/tests/snapshots/vector_pdf_canonical.json

@@ -2,9 +2,30 @@
   "doc_id": "c90fae7576fe514fb08190cb29d1ef5d",
   "source_asset_id": "babe9824b6b28237c0898575a40ba48d",
   "workspace_path": "mojibake.pdf",
-  "title": "mojibake",
+  "title": "untitled",
   "lang": "und",
-  "blocks": [],
+  "blocks": [
+    {
+      "kind": "paragraph",
+      "common": {
+        "block_id": "22bb97fc37da5c55c099e2763f95ffd9",
+        "heading_path": [],
+        "source_span": {
+          "kind": "page",
+          "page": 1,
+          "char_start": 0,
+          "char_end": 64
+        }
+      },
+      "text": "\n<><6E><EFBFBD><EFBFBD><EFBFBD>\u0014<31>\u0000\u0000 <20>=¤̘\u0000  \u0014\u0000 <20> <20><><EFBFBD>T<EFBFBD><54>\u0000 <20><><EFBFBD>L\n<>\\<5C>mŴ\u0000 <20>8ǐ<38>\u0000\u0000 <20>h<EFBFBD><68><EFBFBD><EFBFBD>\u0000 <20><>ư\u0000.\n",
+      "inlines": [
+        {
+          "kind": "text",
+          "text": "\n<><6E><EFBFBD><EFBFBD><EFBFBD>\u0014<31>\u0000\u0000 <20>=¤̘\u0000  \u0014\u0000 <20> <20><><EFBFBD>T<EFBFBD><54>\u0000 <20><><EFBFBD>L\n<>\\<5C>mŴ\u0000 <20>8ǐ<38>\u0000\u0000 <20>h<EFBFBD><68><EFBFBD><EFBFBD>\u0000 <20><>ư\u0000.\n"
+        }
+      ]
+    }
+  ],
   "metadata": {
     "aliases": [],
     "tags": [],
@@ -15,7 +36,9 @@
     "user_id_alias": null,
     "user": {
       "pdf": {
-        "page_count": 0
+        "creator": "anonymous",
+        "page_count": 1,
+        "producer": "ReportLab PDF Library - (opensource)"
       }
     }
   },
@@ -31,7 +54,7 @@
         "at": "1970-01-01T00:00:00Z",
         "agent": "kb-parse-pdf",
         "kind": "parsed",
-        "note": "parser_version=pdf-text-v1; page_count=0"
+        "note": "parser_version=pdf-text-v1; page_count=1"
       }
     ]
   },