feat: warn when a PDF export replaces characters it cannot draw

The hand-written PDF writer uses the standard Courier fonts
(WinAnsiEncoding), so any character outside CP1252 - a Polish, Greek,
Cyrillic or CJK script - was silently written as '?'. Render now
collects the characters it had to replace and returns them, and the
export dialog lists them in a warning once the file is written.

Embedding a TrueType font (e.g. Courier Prime) with a CID/ToUnicode
map would fix the root cause; the warning keeps the zero-dependency
trade-off honest in the meantime.

CHANGELOG.md

@@ -7,6 +7,12 @@ version is in the `VERSION` file.
 
 ## [Unreleased]
 
+### Added
+
+- The PDF export warns when the script contains characters the standard PDF fonts
+  cannot draw: they are written as `?` in the file, and are now listed in a dialog
+  instead of being silently lost.
+
 ### Fixed
 
 - The unsaved-changes marker is now updated atomically: an edit landing while a save