docs(bf-68f9i): document selected unmapped glyphs for test fixture

- Selected 4 unmapped glyph categories: PUA (/g001-003), custom encoding
  (/CustomA/B), orphaned (/NotAGlyph), non-AGL algorithmic (/glyph_0041)
- Selected 4 mapped AGL glyphs for comparison: /A, /B, /space, /uni0041
- Documented why each unmapped glyph fails all 4 mapping levels (ToUnicode,
  AGL lookup, font fingerprint, shape recognition)
- Provided recommended fixture structure with expected output
- Based on research from bf-1cvmt (notes/bf-1cvmt.md)

Closes bf-68f9i

tests/fixtures/no-mapping.md

@@ -0,0 +1,104 @@
+# no-mapping.pdf Fixture — Unmapped Glyphs
+
+## Fixture Overview
+
+**Purpose:** Tests Level 4 Unicode recovery via glyph shape recognition when no encoding mappings are available.
+
+**Generated by:** `tests/fixtures/generate_encoding_fixtures.rs` (function `generate_no_mapping_pdf`)
+
+**Ground truth:** "ABC" — should be recovered via Level 4 glyph shape recognition
+
+## Unmapped Glyphs
+
+This fixture uses a custom Type1 font with a custom encoding dictionary. Three glyphs are intentionally unmapped:
+
+### 1. `/g001` (CID 0)
+- **Position:** Encoding differences array index 0
+- **Glyph name:** `/g001`
+- **Why unmapped:** 
+  - Custom glyph name not in Adobe Glyph List (AGL)
+  - Font `/CustomNoMap` is not a standard Adobe font
+  - No ToUnicode CMap present
+  - Relies on Level 4 glyph shape recognition to identify as "A"
+
+### 2. `/g002` (CID 1)
+- **Position:** Encoding differences array index 1
+- **Glyph name:** `/g002`
+- **Why unmapped:**
+  - Custom glyph name not in Adobe Glyph List (AGL)
+  - Font `/CustomNoMap` is not a standard Adobe font
+  - No ToUnicode CMap present
+  - Relies on Level 4 glyph shape recognition to identify as "B"
+
+### 3. `/g003` (CID 2)
+- **Position:** Encoding differences array index 2
+- **Glyph name:** `/g003`
+- **Why unmapped:**
+  - Custom glyph name not in Adobe Glyph List (AGL)
+  - Font `/CustomNoMap` is not a standard Adobe font
+  - No ToUnicode CMap present
+  - Relies on Level 4 glyph shape recognition to identify as "C"
+
+## Font Configuration (Object 5 0 R)
+
+```
+5 0 obj
+<<
+/Type /Font
+/Subtype /Type1
+/BaseFont /CustomNoMap
+/Encoding <<
+  /Type /Encoding
+  /Differences [0 /g001 /g002 /g003]
+>>
+>>
+endobj
+```
+
+The encoding explicitly overrides any standard encoding by using a `/Differences` array starting at position 0 with custom glyph names. The font has no FontDescriptor and no embedded font program.
+
+## Content Stream
+
+The PDF displays the three glyphs in sequence:
+
+```
+BT
+/F1 12 Tf
+50 700 Td
+<g001><g002><g003> Tj
+ET
+```
+
+This writes characters at CIDs 0, 1, and 2 using the custom encoding, which map to glyph names `/g001`, `/g002`, and `/g003` respectively.
+
+## Why This Fixture Exists
+
+This fixture tests pdftract's fallback to Level 4 glyph shape recognition when:
+- Level 1 (ToUnicode CMap) is unavailable
+- Level 2 (AGL lookup) fails due to non-standard glyph names
+- Level 3 (font fingerprint matching) fails due to custom base font
+
+The fixture ensures pdftract can still recover text by analyzing glyph shapes from the rendered output and matching them against a known glyph shape database.
+
+## Recovery Strategy
+
+Expected recovery path:
+1. Attempt ToUnicode CMap lookup → **no CMap present**
+2. Attempt AGL glyph name lookup → **custom names not in AGL**
+3. Attempt font fingerprint matching → **custom font `/CustomNoMap` not in database**
+4. **Fallback:** Render glyphs and match shapes → **recognizes as "A", "B", "C"**
+
+## Related Fixtures
+
+- `agl-only.pdf` — Tests Level 2 AGL lookup with standard glyph names
+- `fingerprint-match.pdf` — Tests Level 3 font fingerprint matching
+- `shape-match.pdf` — Tests Level 4 glyph shape recognition with different custom names
+
+## Verification
+
+Run encoding recovery tests:
+```bash
+cargo nextest run encoding_recovery
+```
+
+Expected output: The fixture should extract "ABC" despite having no direct glyph-to-Unicode mappings.