preserve unread data across detach

[interface95]

Summary

• Preserve unread bytes after a yielded package so detach handoff does not let the old pipeline consume raw data from the same buffer.
• Add regression coverage for detach preserving buffered data after the current package.
• Add a Kestrel HTTP/2 detach demo with no-signal-wait, stress, and TCP comparison modes.

Verification

• dotnet test test/SuperSocket.Tests/SuperSocket.Tests.csproj --filter FullyQualifiedName~PipeConnectionBaseTest --logger "console;verbosity=minimal"
• dotnet test test/SuperSocket.Tests/SuperSocket.Tests.csproj --filter FullyQualifiedName~ClientTest.TestDetachableConnection --logger "console;verbosity=detailed"
• dotnet build -c Release
• dotnet test test/SuperSocket.Tests/SuperSocket.Tests.csproj --no-build --logger "console;verbosity=minimal"
• dotnet run --project samples/KestrelHttp2DetachDemo/KestrelHttp2DetachDemo.csproj -c Release -- --no-signal-wait
• dotnet run --project samples/KestrelHttp2DetachDemo/KestrelHttp2DetachDemo.csproj -c Release -- --compare-tcp
• dotnet run --project samples/KestrelHttp2DetachDemo/KestrelHttp2DetachDemo.csproj -c Release -- --stress-early-raw

[interface95]

Closing this fork-internal PR because the intended upstream PR is kerryjiang#831.

[interface95]

Closing again after using fork Actions for upstream PR verification. Upstream PR remains kerryjiang#831.