Protect non-embedded CID text layers from PDF/A corruption (closes #1561)

Ghostscript's PDF/A conversion re-embeds non-embedded CID (CJK) fonts by
substituting a system font, which corrupts the character-to-Unicode
mapping and silently destroys an existing text layer -- commonly the OCR
layer Adobe Acrobat adds to scanned CJK documents.

Detect non-embedded CID/Type0 fonts before conversion: with
--output-type auto (the default) downgrade to a regular PDF and preserve
the text layer; with an explicit --output-type pdfa* stop with an error
rather than emit corrupted output. Simple non-embedded fonts (e.g. Latin)
are left alone -- Ghostscript substitutes them without corrupting the
text, and they are far too common to treat as conversion blockers.

Use --output-type pdf to keep the existing text layer, or --force-ocr to
rebuild it with embedded fonts.

docs/releasenotes/version17.md

@@ -5,6 +5,16 @@
 
 ## v17.8.0
 
+- OCRmyPDF no longer silently corrupts a non-embedded CID (CJK) text layer when
+  producing PDF/A ({issue}`1561`). PDF/A requires all fonts to be embedded, so
+  Ghostscript substitutes and re-embeds non-embedded CID fonts — such as the OCR
+  text layer Adobe Acrobat adds to scanned CJK documents — which mangles the
+  text and destroys searchability. OCRmyPDF now detects non-embedded CID fonts
+  before conversion: with `--output-type auto` (the default) it produces a
+  regular PDF and preserves the existing text layer, and with an explicit
+  `--output-type pdfa*` it stops with an error rather than emit corrupted
+  output. Use `--output-type pdf` to keep the text layer, or `--force-ocr` to
+  rebuild it with embedded fonts.
 - Writing the output PDF to standard output (`ocrmypdf input.pdf -`) is now
   protected against corruption at the operating system level. Previously
   OCRmyPDF relied on no in-process code — third-party libraries, plugins, or