docs(bf-4ozvw): analyze no-mapping.pdf fixture structure

Examine the no-mapping.pdf test fixture using PDF analysis tools:
- pdfinfo: extracted metadata (1 page, 660 bytes, PDF 1.4, letter)
- pdffonts: identified CustomNoMap Type 1 font with custom encoding
- Documented structural findings for later reference

Key finding: fixture uses custom encoding with no embedding, designed to
test pdftract's handling of glyphs without standard Unicode mappings.

Closes bf-4ozvw.

notes/bf-4ozvw.md

@@ -0,0 +1,74 @@
+# no-mapping.pdf Fixture Structure Analysis
+
+**Date:** 2026-07-03
+**Bead:** bf-4ozvw
+**Fixture Path:** `tests/fixtures/encoding/no-mapping.pdf`
+
+## PDF Metadata (pdfinfo)
+
+```
+Custom Metadata: no
+Metadata Stream: no
+Tagged:          no
+UserProperties:  no
+Suspects:        no
+Form:            none
+JavaScript:      no
+Pages:           1
+Encrypted:       no
+Page size:       612 x 792 pts (letter)
+Page rot:        0
+File size:       660 bytes
+Optimized:       no
+PDF version:     1.4
+```
+
+## Font and Encoding Information (pdffonts)
+
+```
+name                                 type              encoding         emb sub uni object ID
+------------------------------------ ----------------- ---------------- --- --- --- ---------
+CustomNoMap                          Type 1            Custom           no  no  no       5  0
+```
+
+## Structural Findings
+
+### File Characteristics
+- **Size:** Extremely small (660 bytes) — minimal fixture
+- **Pages:** Single page document
+- **Format:** PDF 1.4 (standard version)
+- **Page Size:** US Letter (612 x 792 pts)
+- **Complexity:** No JavaScript, forms, or metadata streams
+
+### Font Analysis
+- **Font Name:** `CustomNoMap` — descriptive name indicating purpose
+- **Font Type:** Type 1 (PostScript font)
+- **Encoding:** Custom — this is the key characteristic
+- **Embedding:** Not embedded (`emb: no`)
+- **Subsetting:** Not subset (`sub: no`)
+- **Unicode:** No Unicode mapping (`uni: no`)
+
+### Key Observations
+
+1. **Custom Encoding Purpose:** The fixture is specifically designed to test handling of custom font encodings that lack standard mappings to Unicode or other character sets.
+
+2. **Minimal Complexity:** At 660 bytes, this is likely a handcrafted or programmatically generated PDF with minimal content, focused specifically on the encoding edge case.
+
+3. **No Font Embedding:** The font is not embedded, meaning PDF readers would need to have access to the actual font file or rely on the encoding information provided.
+
+4. **Testing Utility:** This fixture appears to be designed to test pdftract's handling of:
+   - Custom encodings that don't map to standard character sets
+   - Glyph-to-text extraction fallback strategies
+   - Error recovery when encoding mappings are unavailable
+
+### Technical Significance
+
+This fixture is particularly valuable for testing:
+- Font encoding edge cases
+- Glyph recognition without standard mappings
+- Error handling in the text extraction pipeline
+- Fallback strategies when standard encoding tables (AGL, CJK, etc.) don't apply
+
+## Notes for Documentation
+
+The fixture name "no-mapping" combined with the custom encoding suggests this is intentionally testing the case where pdftract must handle glyphs that have no direct mapping to readable text. This is a critical edge case for robust PDF text extraction.