[Mluo2011]

Fixes #2372.

FreeText.__init__ set /DA, /DS, /C and /BS but never built an appearance
stream (/AP). Viewers that don't synthesize one — notably PDFium, which Chrome
uses — draw the annotation box but not the text.

This adds a _free_text_appearance_stream() helper that builds a normal appearance
stream (/AP /N) from the annotation's background, border, colour, size and
bold/italic settings, and add_annotation now stores it as an indirect object (a
stream can't sit directly inside the annotation dict).

Test tests/test_annotations.py::test_free_text__appearance_stream fails on the
unpatched code with KeyError: '/AP' and passes after; test_annotations.py is
52/52. Rendered with PDFium the text now shows.

Known limitations (happy to address if preferred):

• the font= argument is drawn as standard Helvetica; text is not wrapped;
• characters outside cp1252 are replaced with ?.

Prepared with help from an AI coding assistant; I reviewed and tested it locally.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.81%. Comparing base (f5b3f07) to head (c373f52).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4358   +/-   ##
=======================================
  Coverage   98.81%   98.81%           
=======================================
  Files          59       59           
  Lines       12139    12174   +35     
  Branches     2310     2317    +7     
=======================================
+ Hits        11995    12030   +35     
  Misses         70       70           
  Partials       74       74           

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