docs(pdftract-5nbp): add verification note for /Differences overlay handler

The /Differences overlay handler was already fully implemented.
All 28 encoding tests pass.

Acceptance criteria:
- [PASS] [ 39 /quotesingle 96 /grave ] parses correctly
- [PASS] [ 39 /a /b /c ] consecutive assignment works
- [PASS] Overlay precedence over base encoding
- [PASS] Unknown glyph names returned for L3/L4 fallback

notes/pdftract-5nbp.md

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+# pdftract-5nbp: /Differences overlay handler verification
+
+## Summary
+
+The `/Differences` overlay handler for font encodings was already fully implemented in `crates/pdftract-core/src/font/encoding.rs`. No code changes were required.
+
+## Implementation details
+
+### `DifferencesOverlay` struct (lines 128-234)
+
+- `entries: Vec<(u8, Arc<str>)>` - sparse list of (code, glyph_name) overrides
+- `parse()` method walks the array linearly, maintaining a cursor that resets on each integer
+- `get()` method performs linear search (fine for <32 entries)
+
+### `FontEncoding` struct (lines 236-364)
+
+- Combines base encoding with differences overlay
+- `parse_from_font()` handles all encoding indirection patterns
+- `glyph_name_for()` lookup order: differences first, base second
+
+## Acceptance criteria verification
+
+### PASS: `[ 39 /quotesingle 96 /grave ]` parsing
+Test: `test_differences_overlay_parse_simple`
+- Code 39 → "quotesingle"
+- Code 96 → "grave"
+
+### PASS: `[ 39 /a /b /c ]` consecutive assignment
+Test: `test_differences_overlay_parse_consecutive`
+- Code 39 → "a"
+- Code 40 → "b"
+- Code 41 → "c"
+
+### PASS: Overlay precedence over base encoding
+Tests: `test_font_encoding_glyph_name_with_differences`, `test_font_encoding_lookup_order`
+- Overlaid codes return the override
+- Non-overlaid codes return the base encoding entry
+
+### PASS: Unknown glyph names returned as-is
+Test: `test_font_encoding_unknown_glyph_name`
+- Arbitrary glyph names not in AGL are returned (not None)
+- Resolver can attempt L3/L4 fallback with the name
+
+## Critical considerations
+
+### PASS: Multiple Differences blocks
+Test: `test_differences_overlay_parse_multiple_blocks`
+- Handles `[ 39 /a /b 100 /x /y ]` correctly
+
+### PASS: Out-of-range code clamping with diagnostics
+Tests: `test_differences_overlay_out_of_range_positive`, `test_differences_overlay_out_of_range_negative`
+- Codes < 0 clamped to 0
+- Codes > 255 clamped to 255
+- Emits `DiagCode::FontEncodingDifferenceOutOfRange` diagnostic
+
+### PASS: Implicit base encoding
+- `FontEncoding::parse_from_font()` handles `/Differences` without `/BaseEncoding`
+- Falls back to `default_base` parameter (Standard for Type1, built-in for TrueType)
+
+## Test results
+
+```
+running 28 tests
+test result: ok. 28 passed; 0 failed; 0 ignored
+```
+
+## Files
+
+- `crates/pdftract-core/src/font/encoding.rs` - Full implementation
+- `crates/pdftract-core/src/font/mod.rs` - Re-exports `FontEncoding`, `DifferencesOverlay`, `NamedEncoding`
+
+## No code changes required
+
+The implementation was already complete and tested. This bead verified existing functionality.