[LoopedBard3]

Summary

Preemptively add .NET 12 support, following dotnet#2911 and including the composite ReadyToRun surface introduced in dotnet#2967. Based on the latest upstream master (9723646).

• Add .NET 12 runtime moniker constants, cached runtime instances and cached toolchains for CoreCLR, NativeAOT, Mono, composite R2R, Mono WebAssembly and Mono WebAssembly AOT.
• Advance CoreRuntime.Latest, the CoreRun fallback TFM, and local NativeAOT build defaults to .NET 12; add the dotnet12 package feed.
• Add net12.0 choices to the C#, F# and VB templates without changing their default target frameworks.
• Cover dotted and compact runtime spellings, cached instances, platform-specific TFMs, CLI toolchain selection, and generated target-framework settings. The experimental CoreCLR WebAssembly parser/toolchain already accepts arbitrary versions and is covered for 12.0 without adding a public experimental constant.

Current upstream already generalizes runtime parsing, MSBuild framework error mapping, and dotMemory/dotTrace support by runtime type/version, so those former per-version switch tables do not need new branches.

Validation

Targeted runtime detection/parsing, CLI, project generator and dotMemory/dotTrace tests:

• net10.0: 313 passed, 2 skipped.
• net472: 308 passed, 7 skipped, including the non-Core host fallback.
• All three template JSON files parse and expose net12.0; the dotnet12 feed responds successfully.

No .NET 12 SDK is installed locally, so actual .NET 12 benchmark compilation/execution (including NativeAOT, R2R and Wasm workloads) is not yet validated. Kept as a draft for that follow-up.

[LoopedBard3]

Closing this draft because the PR creation tool targeted the fork instead of dotnet/BenchmarkDotNet. The feature branch is retained for the upstream draft.