docs: record the Adobe licence covering the vendored CMaps

The CMaps come from Adobe, not Mozilla, and are not covered by the Apache-2.0
grant that applies to the rest of PDF.js.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

THIRD_PARTY_LICENSES.md

@@ -24,6 +24,12 @@ Redistributed files:
 The upstream package ships no `NOTICE` file, so the attribution requirement of
 Apache-2.0 section 4(d) does not apply.
 
+### CMaps inside `vendor/cmaps/`
+
+The character maps are Adobe's, not Mozilla's, and carry their own BSD-3-Clause
+terms: © 1990–2009 Adobe Systems Incorporated. The text ships with them in
+`vendor/cmaps/LICENSE` and must stay in place.
+
 ### Fonts inside `vendor/standard_fonts/`
 
 The replacement fonts are not covered by Apache-2.0 and carry their own terms.