Migrate UnregisterAssembly to multithreaded execution

## Background
`UnregisterAssembly` reverses `RegisterAssembly` by unregistering managed assemblies and their type libraries from COM. Like its counterpart, it is `AppDomainIsolatedTaskExtension`, gated on `NETFRAMEWORK && FEATURE_APPDOMAIN`.

## Migration scope
- [ ] Apply `[MSBuildMultiThreadableTask]` (defensive)
- [ ] Implement `IMultiThreadableTask` only if 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

## Known complexity factors
- AppDomain isolation; `TaskEnvironment` must cross the AppDomain boundary
- Native COM unregistration; mutates registry (`TYPE_E_REGISTRYACCESS` handling)
- NETFRAMEWORK-only — multithreaded mode unlikely to ever exercise this code
- Globally serialized in practice due to registry mutation

## 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: #11834
- Migration skill: .github/skills/multithreaded-task-migration/SKILL.md