docs(bf-3s7gd): document CMAP skip check verification (already implemented)

The CMAP skip check for unmapped glyphs was already implemented in
commit 7660cfc8 (bead bf-4zgpr). This note documents the verification
of that implementation.

Acceptance criteria verified:
- Code checks glyph name against unmapped_glyph_names ✓
- Unmapped glyphs are skipped during CMAP generation ✓
- Skip happens cleanly (no errors, no panics) ✓
- Code compiles without errors ✓

See notes/bf-3s7gd.md for details.

notes/bf-3s7gd.md

@@ -0,0 +1,86 @@
+# bf-3s7gd: CMAP skip check for unmapped glyphs
+
+## Status: ALREADY IMPLEMENTED
+
+This bead's work was already completed in a previous iteration. The skip logic was implemented in commit `7660cfc8` (bead `bf-4zgpr`), which refactored an earlier implementation.
+
+## Implementation location
+
+File: `crates/pdftract-core/src/font/encoding.rs`
+
+Lines 224-229:
+```rust
+// Skip unmapped glyph names (e.g., .notdef) to prevent them from
+// appearing in text extraction output. These glyphs have no valid
+// Unicode mapping and should emit GLYPH_UNMAPPED diagnostics instead.
+if !overlay.is_unmapped_glyph_name(&name) {
+    overlay.entries.push((cursor as u8, Arc::clone(name)));
+}
+```
+
+## Supporting infrastructure
+
+1. **Unmapped glyph names configuration** (`unmapped.rs`):
+   - `is_unmapped_glyph_name(name: &str) -> bool` - checks against global UNMAPPED_GLYPH_NAMES set
+   - Handles glyph names with or without leading `/`
+
+2. **DifferencesOverlay instance method** (`encoding.rs` lines 274-282):
+   ```rust
+   fn is_unmapped_glyph_name(&self, name: &str) -> bool {
+       let clean_name = if name.starts_with('/') {
+           &name[1..]
+       } else {
+           name
+       };
+       self.unmapped_glyph_names.contains(clean_name)
+   }
+   ```
+
+3. **Configuration field** (`encoding.rs` lines 136, 164-168):
+   - `unmapped_glyph_names: HashSet<String>` - stores the set of glyph names to skip
+   - Defaults to global `UNMAPPED_GLYPH_NAMES` set
+   - Can be customized via `with_unmapped_glyph_names()` constructor
+
+## Acceptance criteria verification
+
+- ✅ **Code checks glyph name against unmapped_glyph_names before CMAP entry creation**
+  - Lines 227: `if !overlay.is_unmapped_glyph_name(&name)`
+
+- ✅ **Unmapped glyphs are skipped during CMAP generation**
+  - Lines 227-229: If glyph is unmapped, `entries.push()` is skipped
+
+- ✅ **Skip happens cleanly (no errors, no panics)**
+  - Tests pass: `test_differences_overlay_skips_notdef`, `test_differences_overlay_skips_notdef_with_slash`
+  - No diagnostics emitted for skipped glyphs
+
+- ✅ **Code compiles without errors**
+  - `cargo test -p pdftract-core --lib differences_overlay_skips_notdef` passes
+
+## Test results
+
+```bash
+$ cargo test -p pdftract-core --lib differences_overlay_skips_notdef --no-fail-fast
+running 2 tests
+test font::encoding::tests::test_differences_overlay_skips_notdef ... ok
+test font::encoding::tests::test_differences_overlay_skips_notdef_with_slash ... ok
+
+test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 2895 filtered out
+```
+
+## Related beads
+
+- `bf-4zgpr`: Added `unmapped_glyph_names` config field to DifferencesOverlay
+- `bf-6cvmz`: Documented unmapped_glyph_names configuration structure
+- `bf-o4nrq`: Documented CMAP entry creation point
+- `bf-1puzp`: Earlier verification of CMAP skip logic
+
+## CMAP entry creation point
+
+This implementation is at the CMAP entry creation point identified in child beads:
+- **Marker comment**: Line 222-223 in `encoding.rs`
+- **Context**: Type1 font encoding differences parsing
+- **Call site**: `DifferencesOverlay::parse()` method
+
+## Conclusion
+
+All acceptance criteria for this bead were already met by prior work. The implementation correctly skips unmapped glyph names during CMAP generation, preventing them from appearing in text extraction output.