Document and clarify _imagingft

[akx]

Refs #9898 - had to figure out what's happening in _imagingft to get to the bottom of the kerning woes, and these came out of that. The actual bugfixes will be in another PR stacked on this.

Basically:

• there was some type confusion with FreeType's 26.6 fixed-point values; some comments noted values were in 26.6 when they are in pixels, or the other way around. This uses FT's FT_F26Dot6 type (an alias to long, since C doesn't have real newtypes, more's the shame) to make it more obvious to the reader.
• adds macros to work with said data type, to avoid unclear * 64s or << 6s. Not truly type-safe since, well, see above about newtypes, but better than throwing magic numbers around.
• adds doc comments for the internal functions, because the calling conventions weren't clear (more on that a couple bullet points later)
• fixes _imagingft.pyi's types to match the reality of the module, and adds tests that prove that the typings were wrong.
• changes the core getfont()'s signature to make all arguments required (C interface is internal #9879 in mind, that should be fine to do), since calling getfont() without encoding could handily segfault (since encoding in C was left uninitialized, set to stack garbage).

[radarhere]

adds tests that prove that the typings were wrong

Looks to me like test_bytearray_not_supported() passes without the rest of your changes.

[akx]

Er, yes..? The test passes (even on main) because font.getlength, font.getbbox, font.getmask don't actually accept bytearrays. If you remove the pytest.raises, it will fail.

On main, trying to pass in a bytearray fails:

$ uv pip install -e .
$ uv run python
Python 3.14.6 (main, Jun 11 2026, 03:55:33) [Clang 22.1.3 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from PIL import ImageFont
>>> f = ImageFont.load_default()
>>> f.font.getsize(bytearray(b"f"))
Traceback (most recent call last):
  File "<python-input-3>", line 1, in <module>
    f.font.getsize(bytearray(b"f"))
    ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
TypeError: expected string or bytes
>>>

even though the typings say they should be accepted:

~/b/Pillow (main) $ grep -C2 getsize src/PIL/_imagingft.pyi
    def getsize(
        self,
        string: str | bytes | bytearray,

[radarhere]

Oh, right, most PR tests fail without the other changes, but that was not your intention here.