Flip the default of UseVulkanOnWin32, UseVulkanOnX11 and
UseVulkanOnSkiaAndroid to true. Each rendering path already falls back to
OpenGL/OpenGL ES and then software when Vulkan is unavailable, so the new
default degrades gracefully. Update the Win32/X11 RenderingBackend.Default
enum docs to reflect the Vulkan-first order.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Replace the temporary CS0618 pragma suppressions with a real migration to
the SkiaSharp 4 immutable path model:

- Path construction/mutation now goes through SKPathBuilder; the built path
  is obtained via Detach() (fresh) or Snapshot() (copy). Pooled/reusable
  paths pool the builder and Reset() it between uses.
- Surviving SKPath operations (Op, Transform, Offset, Reset) are kept; only
  the deprecated construction methods are moved off. Rewind() -> Reset().
- The renderer's clip plumbing keeps its out-param/scratch signatures: a
  Transform(identity, dst) helper (CopyPath/SetPathToRect) overwrites an
  existing path with an exact copy, replacing the deprecated dst.AddPath/
  AddRect re-seeding while preserving verbs and fill type.
- SKPaint.GetFillPath now fills an SKPathBuilder; SKCanvas.DrawBitmap/
  DrawImage take SKSamplingOptions; SKCanvas.DrawText takes SKTextAlign;
  SKTypeface.ContainsGlyph -> SKFont.ContainsGlyph.

Builds clean (0 warnings/errors, TreatWarningsAsErrors) for the full
SamplesApp.Skia.Generic plus the Win32 and X11 hosts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

When a resolved typeface is a variable font (exposes OpenType variation
axes), instantiate it at the requested style via SKTypeface.Clone with an
SKFontArguments variation position, mapping the WinUI font properties onto
the standard axes:
- wght <- FontWeight.Weight (exact numeric weight, e.g. 350/650)
- wdth <- FontStretch (as a width percentage)
- ital <- FontStyle.Italic
- slnt <- FontStyle.Italic/Oblique

Values are clamped to each axis' supported range and axes already at their
default are left untouched; static fonts are returned unchanged. This lets
a single variable font render arbitrary weights/widths instead of snapping
to the nearest static instance. Per-style results stay cached by the
existing FontDetailsCache key.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Use the typeface introspection added in SkiaSharp 4 when resolving fonts:

- Honor the "<file-uri>#<family name>" hint for embedded fonts. When the
  file is a TrueType/OpenType collection (.ttc/.otc), probe its faces and
  pick the one whose family or PostScript name matches the hint instead of
  always loading face 0. The default (no hint / single face) path is
  unchanged and streams directly; probing only happens when face 0 doesn't
  match, reading the data back via SKTypeface.OpenStream/SKStream.GetData,
  and is bounded to guard malformed files.
- Treat an empty typeface (SKTypeface.IsEmpty) from FromFamilyName the same
  as null so resolution falls back to the default font.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

- Replace the CopyPath helper with inline src.Transform(SKMatrix.Identity, dst)
  and share a single CreateRectPath from SkiaExtensions (was duplicated in Visual
  and SkiaRenderHelper); drop the SetPathToRect indirection and a dead Reset() +
  redundant transform on the native-clip path.
- Use SKSamplingOptions.Default instead of default(SKSamplingOptions) on the
  DrawBitmap/DrawImage calls.
- Draw the sample's rounded rect with DrawRoundRect instead of building a path.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

- Select collection faces by probing the font's own SKData; SKTypeface.OpenStream
  ().GetData() returns null here, so the previous approach never found sub-faces
  and every "#family" hint fell back to face 0.
- Carry the face's collection index in SKFontArguments when cloning; otherwise
  Clone() silently drops the weight/width variation for any non-first face.
- Cache loaded font data per source uri so a new FontWeight of the same file
  resolves synchronously instead of momentarily rendering the default font
  (flicker when animating weight on a variable font).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Adds a Fonts sample that loads two families (Inter + Bitter) from a single .ttc
via the "#family" hint and sweeps the wght axis, exercising collection face
selection and variable-weight rendering. Bundles the OFL-licensed collection and
its license.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>