feat(🔤): expose SkFontStyleSet on SkFontMgr

[IsaacInsoll]

feat(🔤): expose SkFontStyleSet on SkFontMgr

Summary

Adds JSI bindings for Skia's SkFontStyleSet, letting JS enumerate the exact
variants a font family ships with. Two new methods on SkFontMgr:

• matchFamily(name: string): SkFontStyleSet | null
• createStyleSet(index: number): SkFontStyleSet | null

And a new host object, SkFontStyleSet, exposing:

• count()
• getStyle(index) → FontStyleEntry ({ weight, width, slant, name })
• createTypeface(index)
• matchStyle(style)
• dispose()

Motivation

matchFamilyStyle is the only JS-side entry point for family lookup today, and
it has two limitations:

1. It silently falls back across families when the requested family doesn't
   exist — there is no way to distinguish "Helvetica Bold" from "some other
   font's Bold that Skia picked for you".
2. It cannot enumerate what a family actually ships. Building a variant picker
   (weight/width/slant) currently requires bundling the font metadata yourself.

SkFontStyleSet is the native Skia primitive that answers both questions. It's
already returned by SkFontMgr::matchFamily / SkFontMgr::createStyleSet on
both platforms — this PR just lifts it into JS.

API

interface SkFontMgr {
  // ...existing
  createStyleSet(index: number): SkFontStyleSet | null;
  matchFamily(name: string): SkFontStyleSet | null;
}

interface FontStyleEntry extends Required<FontStyle> {
  name: string; // PostScript name, e.g. "Helvetica-BoldOblique"
}

interface SkFontStyleSet extends SkJSIInstance<'FontStyleSet'> {
  count(): number;
  getStyle(index: number): FontStyleEntry;
  createTypeface(index: number): SkTypeface | null;
  matchStyle(style: FontStyle): SkTypeface | null;
}

FontStyleEntry reuses the existing FontWeight / FontWidth / FontSlant
enums via Required<FontStyle> for type consistency with the rest of the font
API.

matchFamily resolves family aliases ("System" on iOS, etc.) via
RNSkPlatformContext::resolveFontFamily — same behavior as matchFamilyStyle
after #3603.

Implementation notes

• New C++ host object: cpp/api/JsiSkFontStyleSet.h. Follows the
  JsiSkTypeface pattern: EXPORT_JSI_API_TYPENAME, static toValue,
  getMemoryPressure, getObjectType, dispose inherited from the base.
• JsiSkFontMgr.h gains createStyleSet / matchFamily as thin wrappers and
  re-uses resolveFontFamily for alias resolution.
• Nullable returns match Skia's C++ contract: an unknown family / out-of-range
  index returns jsi::Value::null() rather than throwing.
• Web: stub JsiSkFontStyleSet that throws "Not implemented on React Native
  Web", parallel to how matchFamilyStyle is stubbed today. CanvasKit has no
  FontStyleSet equivalent, so web callers should guard with Platform.OS.

Testing

• yarn lint — clean
• yarn tsc — clean
• yarn test — existing suite passes. No new unit tests yet — see the
  "Follow-ups" note below.
• Manually verified against the system font manager on iOS and Android by
  enumerating a known-multi-variant family (Helvetica on iOS, sans-serif on
  Android) and confirming count(), getStyle(i).name, and
  createTypeface(i) return sensible values.

No yarn e2e run yet either — e2e requires a rendering assertion, and
SkFontStyleSet doesn't draw. Open to suggestions on the right harness for
this; happy to add tests in whatever form you prefer before merge.

Docs

New page: apps/docs/docs/text/font-style-set.md (included in this PR) with
usage examples for matchFamily, createStyleSet, and a "pick the closest
variant" worked example. Linked from the sidebar under Text.

Checklist

• Types updated (src/skia/types/Font/FontMgr.ts + generated .d.ts)
• Native binding added and compiles on iOS and Android
• Web stub implemented and throws with a clear message
• Unit tests — not yet, happy to add once we agree on the surface
• E2E test — not yet, see Testing notes
• Docs page
• No breaking changes — purely additive

Notes for reviewers

• Tests: I'd like guidance before writing them. Unit tests in
  packages/skia/src/skia/__tests__/ seem to target pure JS; I'm not sure
  what the preferred way is to cover a JSI host object that wraps native font
  data. Pointers welcome.
• Should SkFontStyleSet expose a [Symbol.iterator] / convenience helpers in
  JS (e.g. getStyles(): FontStyleEntry[])? I kept the surface minimal and
  Skia-faithful; happy to add an idiomatic JS helper in a follow-up.
• getMemoryPressure returns 2048 to match JsiSkFontMgr. A style set is
  tiny; open to tuning this if you have a preferred heuristic.