ENH: Add paragraph direction to FT2Font layout - #32563

PR summary

Part of #32511.

Matplotlib never tells libraqm the paragraph direction, so libraqm guesses it from the first
letter with a strong direction. An Urdu or Arabic sentence that starts with an English word,
is then laid out left-to-right and its words and punctuation come out in the wrong order.

This PR does the low-level part we agreed on in the issue:

• a new ft2font.Direction enum (DEFAULT, LTR, RTL, TTB) that maps to raqm_direction_t
• a direction argument on FT2Font.set_text and FT2Font._layout, passed to
  raqm_set_par_direction
• the direction is part of the layout cache key, so a cached left-to-right layout is not reused
  for right-to-left

The new test test_layout_direction lays out "Python اردو" and "اردو Python" with each direction
and checks which side the English word ends up on.

AI Disclosure

I used Claude to explain the relevant code to me.
Wrote the the C++ changes and the test cases, following the approach from the issue discussion.
Ran the following tests locally:

• test_ft2font, test_text, mathtext and the agg
• pdf, ps and svg backend tests

PR quality check

• Use an expressive title
• New and changed code is tested
• Plotting related features are demonstrated in an example -> N/A for now, will add with the Text API
• New features and API changes have release notes -> N/A for now, will add with the Text API
• Documentation complies with general and docstring guidelines