Fix #3817: avoid re-entrant singleton cctor NullReferenceException

[mattleibow]

Fixes #3817

Root cause

SKTypeface's static constructor built its default typeface by reading the managed singletons SKFontManager.Default.Handle and SKFontStyle.Normal.Handle. Those are SKObject types whose own static constructors participate in the same eager-initialization cascade driven by SKObject.EnsureStaticInstanceAreInitialized().

When SKFontManager or SKFontStyle was the first SkiaSharp type touched in a process, reading them from SKTypeface's cctor re-entered a static constructor that was still running on the current thread. The CLR allows a thread to re-enter a type initializer it is already running, so the read observed a not-yet-assigned (null) singleton field and threw NullReferenceException → surfaced as TypeInitializationException (e.g. on SKFontManager.Default). This is the chronic main CI crash (Windows/Linux .NET Core test jobs).

Fix

Build the default typeface directly from raw native handles (sk_fontmgr_create_default / sk_fontstyle_new / sk_fontmgr_legacy_create_typeface) inside SKTypeface's cctor, in a try/finally that releases them. This severs both reads of the vulnerable managed singletons, so the cctor no longer depends on any other singleton being fully constructed.

• Behavior-equivalent: SKFontStyle.Normal == sk_fontstyle_new(Normal=400, Normal=5, Upright).
• Native ref-counting unchanged: sk_fontmgr_create_default returns +1; legacy_create_typeface does not retain the font manager; both temporaries are released in finally.
• ABI: additive/internal only — no public surface change.

Regression test

Static constructors run once per process, so an in-process xUnit test in an existing assembly can't reproduce "type X touched first" — something always touches SKObject before the test body runs.

Added a small dedicated test assembly, SkiaSharp.Tests.SingletonInit.Console, modeled on SkiaSharp.Vulkan.Tests.Console. dotnet test launches a separate test-host process per assembly, so this assembly's single [Fact] is the first SkiaSharp code touched in its process. The test accesses SKFontManager.Default first and asserts it — plus SKFontStyle.Normal.Handle and SKTypeface.Default.Handle — are non-zero, reproducing the exact #3817 first-touch ordering with no child processes or copy targets.

The assembly is wired into the cake test runners (tests-netcore.cake for net10.0, tests-netfx.cake for net48) and tests/SkiaSharp.Tests.Console.sln, so it builds and runs on every CI leg including .NET Framework.

Verification

• Reproduced the NRE deterministically on clean main: with the fix reverted, the new test fails with exit code 1 and the exact chain SKFontManager..cctor → SKObject..cctor → SKTypeface.EnsureStaticInstanceAreInitialized() → SKTypeface..cctor. With the fix it passes, exit code 0.
• New assembly (net10.0): Passed 1/1, process exit code 0.
• The reverted console test project rebuilds clean; full console suite remains green.

[github-actions]

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