[WIP] Modernize Splat multi-TFM via OS-aware MSBuild

[Copilot]

• Analyze current Directory.Build.props and project structure
• Create centralized TFM buckets with OS-aware conditions in src/Directory.Build.props
• Add System.Net.Http package reference fix for net462/net472
• Configure tests/benchmarks to use modern .NET targets automatically
• Test build functionality on current environment
• Verify all projects can reference centralized properties
• Update any project files that need to consume new centralized properties
• Add contributor guidelines for the new TFM approach

Key Changes Planned:

• Centralize all TFM logic in src/Directory.Build.props using $([MSBuild]::IsOsPlatform(...)) conditions
• Create property buckets: SplatModernTargets, SplatCoreTargets, SplatNetFxTargets, etc.
• OS-aware composition for Windows (.NET Framework + Windows-specific), macOS (Apple TFMs), Linux (Core + Android)
• Automatic test/benchmark TFM detection to use modern .NET instead of netstandard
• System.Net.Http version pinning for net462/net472 to resolve compile conflicts

Environment Note: Current SDK is .NET 8.0.119, so will use net8.0 for modern targets until .NET 9.0 SDK is available.

Fixes #1400.

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[github-actions]

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