Add .NET Framework SDK support

[nurfed1]

Summary

• add an opt-in --netfxsdk-version option that selects and downloads the matching .NET Framework SDK package
• splat NetFxSDK headers and architecture-specific libraries in both regular and /winsysroot layouts
• include NetFxSDK files in minimization maps while remaining compatible with existing map files
• preserve the existing public pruning and execution entry points, and leave default output unchanged when NetFxSDK is not requested

Validation

Tested with Rust 1.98.0 / LLVM 22.1.8:

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --lib --bin xwin
• cargo test --release verify_compiles -- --test-threads=1
• cargo test --release verify_deterministic -- --test-threads=1
• cargo publish --dry-run --allow-dirty

Also verified package selection with VS 16 (4.8) and the current manifest (4.8.1), regular and winsysroot splats, minimized-map replay, and compilation/linking of a metahost.h / mscoree.lib probe importing CLRCreateInstance.