Fix duplicate faces when loading fonts from a directory

[ChrisJr404]

Fixes #683.

load_fonts_from_paths kept a single families map for the whole scan and never cleared it, then re-merged the entire map into self.families on every callback, so each face got re-added on later callbacks. On top of that it re-scanned the whole file with scan_memory even though scan_paths had already parsed the font, which multiplied the duplicates further for collection (.ttc) files.

This registers the font that scan_paths already handed us directly and gives each callback its own accumulator, so every face lands exactly once. register_fonts still scans the memory blob it is given, but now goes through the same per-font registration path.

Added a regression test that loads the system font directory and checks that no family ends up with a duplicate face (same source and index). It is gated the same way as the existing make_shared_matches_local test and returns early when no font directory is present.

[DJMcNab]

Welcome! Unfortunately this code is a little bit outside of my wheelhouse to review quickly.

However, I'd just like to understand your workflow here - we have a recently introduced PR template, which you haven't followed. What tool did you use the open this PR? It might be worth reporting to them that they didn't surface the PR template properly