docs(bf-4g7gb): document unmapped glyph test fixture verification

notes/bf-4g7gb.md

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+# bf-4g7gb: Create unmapped glyph test fixture
+
+## Task
+Create a PDF test fixture containing glyphs without valid Unicode mappings that trigger the FONT_GLYPH_UNMAPPED diagnostic.
+
+## Finding
+The fixture already exists at `tests/fixtures/encoding/no-mapping.pdf` and meets all acceptance criteria.
+
+## Acceptance Criteria - ALL MET
+
+### ✅ Test fixture PDF exists in tests/fixtures/
+- Location: `tests/fixtures/encoding/no-mapping.pdf`
+- Size: 660 bytes
+- SHA256: `b24f88d3add958bfec1d6b134f2cd030cd41bb1932bedbe99405599bd01fa8f0`
+
+### ✅ Fixture contains at least one glyph without a Unicode mapping
+- Contains **three unmapped glyphs**: /g001, /g002, /g003
+- Custom encoding differences array: `[0 /g001 /g002 /g003]`
+- No ToUnicode CMap
+- No standard encoding
+- Non-AGL glyph names (not in Adobe Glyph List)
+- Content stream uses byte codes `<000102>` to reference the three glyphs
+- Expected extraction: U+FFFD U+FFFD U+FFFD (three replacement characters)
+
+### ✅ Fixture is documented with expected unmapped glyphs
+- Documented in `tests/fixtures/PROVENANCE.md` (line 199-208):
+  ```
+  # encoding/no-mapping.pdf
+  Generated by tests/fixtures/generate_unicode_recovery_fixtures.rs
+  PDF 1.4, Type1 font with custom glyph names, no ToUnicode CMap, no standard encoding
+  Level 4 Unicode recovery test fixture (worst case: no encoding fallback)
+  Content: <20><><EFBFBD> (three U+FFFD replacement characters - glyph names /g001/g002/g003 are not in AGL and cannot be recovered)
+  ```
+- Generator code at `tests/fixtures/encoding/generate_unicode_recovery_fixtures.rs:68-98` with clear comments:
+  ```rust
+  /// no-mapping.pdf: Custom encoding, no ToUnicode, no standard encoding
+  /// Tests Level 4 worst-case: No ToUnicode, no AGL match, no fingerprint match
+  ```
+- Ground truth file: `tests/fixtures/encoding/no-mapping.txt` containing `<60><><EFBFBD>` (9 bytes UTF-8)
+
+## Additional Verification
+
+### FONT_GLYPH_UNMAPPED Diagnostic
+- Diagnostic code: `FONT_GLYPH_UNMAPPED`
+- Definition: `crates/pdftract-core/src/diagnostics.rs:17` (DiagCode::FontGlyphUnmapped)
+- Emission logic: `crates/pdftract-core/src/font/resolver.rs:24-26`
+  ```rust
+  /// Emit the GLYPH_UNMAPPED diagnostic exactly once per (font, code) miss.
+  ```
+- JSON output test: `crates/pdftract-core/src/output/json.rs:369-386`
+  ```rust
+  assert_eq!(error_json[0].code, "FONT_GLYPH_UNMAPPED");
+  assert_eq!(error_json[0].severity, "error");
+  ```
+
+### Integration Tests
+- Encoding recovery test: `crates/pdftract-core/tests/encoding_recovery.rs:210-223`
+  ```rust
+  #[test]
+  fn test_no_mapping_fixture() {
+      let fixture = Fixture {
+          name: "no-mapping".to_string(),
+          pdf_path: PathBuf::from(format!("{}/no-mapping.pdf", FIXTURES_DIR)),
+          ground_truth_path: PathBuf::from(format!("{}/no-mapping.txt", FIXTURES_DIR)),
+      };
+      let (_, recovery_rate, _) = test_fixture(&fixture);
+      assert!(recovery_rate >= MIN_RECOVERY_RATE, ...);
+  }
+  ```
+- Used throughout codebase by 40+ test and debug files (grepped from codebase)
+
+## PDF Structure
+```
+%PDF-1.4
+1 0 obj: Catalog → /Pages 2 0 R
+2 0 obj: Pages → /Kids [3 0 R], /Count 1
+3 0 obj: Page → /Parent 2 0 R, /MediaBox [0 0 612 792], /Resources << /Font << /F1 5 0 R >> >>, /Contents 4 0 R
+4 0 obj: Content stream → BT /F1 12 Tf 50 700 Td <000102> Tj ET (byte codes for glyphs /g001, /g002, /g003)
+5 0 obj: Font → /Type /Font /Subtype /Type1 /BaseFont /CustomNoMap /Encoding << /Type /Encoding /Differences [0 /g001 /g002 /g003] >>
+```
+
+## Conclusion
+All acceptance criteria are fully met by the existing `no-mapping.pdf` fixture. No additional work required.
+
+## Related
+- Bead bf-512z1: Unicode recovery Phase 2 exit gate (≥90% recovery rate)
+- Bead bf-f0xqd: Corrected no-mapping.pdf ground truth from "ABC" to U+FFFD
+- Bead bf-1m30m: Regenerated no-mapping.pdf via generate_unicode_recovery_fixtures.rs
+- Plan Section TH-03: Test harness for unmapped glyph diagnostics