Adds a FreeType-based Discover implementation for Windows that walks
the system (C:\Windows\Fonts) and per-user
(%LOCALAPPDATA%\Microsoft\Windows\Fonts) font directories, matching
descriptors via family_name / SFNT name table and optionally codepoint
presence.
Wired up as a new .freetype_windows backend which Backend.default() now
returns on Windows. Existing freetype-only paths are untouched.
With this in place, standard code paths -- +list-fonts, SharedGridSet
font-family lookup, CodepointResolver fallback -- work on Windows
without any os.tag == .windows branches in the caller.
Co-authored-by: Claude <noreply@anthropic.com>
This continues our work to improve the amount of explicit error sets
we use in the codebase. Explicit error sets make it easier to understand
possible failure scenarios, allow us to use exhaustive matching, create
compiler errors if errors are unexpectedly added or removed, etc.
The goal eventually is 100% coverage but we're not even close yet.
This just moves us a little closer.
