Don't emit unnecessary unchecked scope for native-int literals

[tannergooding]

Fixes #709.

A bare integer literal whose target type resolves to a native-sized integer (nint/nuint) was wrapped in an unchecked scope whenever its value fell outside the 32-bit range, even in expression contexts where the literal keeps its natural C# type and is never narrowed.

The root cause is that on an LP64 target unsigned long maps to nuint, so the 18446744073709551615UL literal in SDL_size_add_check_overflow is treated as nuint and IsUnchecked applies the nuint 32-bit-range check (> uint.MaxValue) to it. But the narrowing (nuint)/(nint) cast -- the only thing that would actually overflow on a 32-bit target -- is only emitted for VarDecl initializers (see the IsPrevContextDecl<VarDecl> gate in UncheckStmt). In a plain expression the literal just keeps its natural ulong type, so no unchecked is needed.

This restricts the native-int range check in the IntegerLiteral path of IsUnchecked to the VarDecl context, which is symmetric with the existing cast-emission gate:

• SDL_size_add_check_overflow (expression) now emits if (b > (18446744073709551615U) - a).
• SIZE_MAX and friends (VarDecl init) still emit unchecked((nuint)(18446744073709551615U)).

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The existing CLongDefinesRegressionTestUnix baseline test (added in #699 to track this) is reused as the regression; its expected output is updated to the ideal (unchecked removed) and the now-stale "not ideal yet" comments are refreshed. The case is inherently Unix-only -- it requires the unsigned long to nuint mapping -- so it keeps its [Platform("unix")] gate.

Verified with dotnet build -c Release (0 warnings) and dotnet test -c Release (generator suite: 3770 passed, 0 failed). The Unix-gated regression was also confirmed by temporarily un-gating it and running against the Unix-typed baseline harness on a Windows host.