Migrate RegisterAssembly to multithreaded execution

[jankratochvilcz]

Background

RegisterAssembly registers a managed assembly for COM interop, optionally producing a type library. It is currently AppDomainIsolatedTaskExtension, gated on NETFRAMEWORK && FEATURE_APPDOMAIN, so it only ever runs in the legacy single-threaded NET Framework host.

Migration scope

• Apply [MSBuildMultiThreadableTask] (defensive — task only runs on .NET Framework)
• Implement IMultiThreadableTask only if any code path is reachable from a multithreaded host
• Absolutize paths via TaskEnvironment.GetAbsolutePath() for: Assemblies items, TypeLibFiles items, AssemblyListFile
• Use TaskEnvironment.GetEnvironmentVariable() for: N/A
• Use TaskEnvironment.GetProcessStartInfo() for: N/A (uses in-proc registration, not a child process)

Known complexity factors

• AppDomain-isolated; Execute runs in a separate AppDomain — TaskEnvironment must round-trip across the boundary
• COM ITypeLibExporterNotifySink callbacks; native registry mutation
• NETFRAMEWORK-only; multithreaded mode is a .NET (Core) host concern, so practical migration value is low
• Mutates global system state (registry) — inherently not parallel-safe regardless of task plumbing

Exit criteria

• Build clean, no new warnings
• Existing tests pass
• No regression in error message paths (Sin 2 audit)
• No leakage of absolutized paths into [Output] properties (Sin 1 audit)

References

• Parent: Migrate Tasks in the MSbuild repo to the new Task type #11834
• Migration skill: .github/skills/multithreaded-task-migration/SKILL.md