fix(input): heading base-font bold + list-item spacing guard

[wildseansy]

Follow-up fixes to #2 (the first landed on the branch just after that merge).

Changes

1. iOS headings derive bold from the base font — a heading with a bold fontWeight (the shared default from headingDefaults) built its font with UIFont systemFontOfSize:weight:, discarding the editor's base font family, so custom-font editors got system-font headings. Android already folds heading weight into the current typeface (InputHeadingSpan); iOS now derives the bold face from the base font via a font-descriptor trait, falling back to the system font only when the family has no bold face. Non-bold explicit weights keep the previous system-font behavior. (Also submitted upstream as fix(input): derive heading bold from the base font on iOS software-mansion/react-native-enriched-markdown#586.)
2. Android list-item spacing guard — StaticLayout treats a LineHeightSpan as paragraph-scoped: chooseHeight runs for every wrapped line of the item regardless of the span's character range, and the FontMetricsInt is reused across those calls, so the unguarded relative ascent adjustment compounded line after line — wrapped list items rendered with progressively growing gaps between visual lines. The adjustment is now guarded to the single line containing the span start, so only the item's first visual line is spaced, exactly once.

Testing

• Both verified in the consuming app (Tonic mouni): headings render in the editor's custom font (SofiaPro) bold face on an iOS simulator; wrapped list items render with uniform line density on an Android emulator (previously growing gaps).
• clang-format clean on touched iOS files.

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