Honor styles in local text typefaces

[Happypig375]

Summary

• Select matching regular, bold, italic, or bold-italic local typefaces for ordinary styled text before Unicode math remapping.
• Keep collection-backed typefaces live between renders while materializing one-shot enumerables once, using one coherent style/fallback snapshot per render.
• Preserve decorative styles, default math behavior, and deterministic glyph fallback.

Validation

• Independent code review: approved with no findings.
• Focused styled-local-typeface tests: 6/6 passed.
• Full rendering suite: 1232/1232 passed.
• Full XAML suite, including live add/remove on Avalonia and MAUI: 18/18 passed.
• Scoped formatter verification and git diff check: passed.
• Release build: passed, including public API analyzers.

Cost and compatibility

Non-collection sources are materialized once. Mutable collections are captured once per render operation so sequential add/remove remains live; callers must not mutate them concurrently with rendering. No public API signatures change.

Fixes #142.