Migrate ResolveSDKReference to multithreaded execution

## Background
`ResolveSDKReference` resolves SDK references against installed Windows / extension SDKs and emits resolved metadata items. It is one of the larger tasks (~75 KB) with substantial caching and Windows-specific logic.

## Migration scope
- [ ] Apply `[MSBuildMultiThreadableTask]`
- [ ] Implement `IMultiThreadableTask`
- [ ] Absolutize paths via `TaskEnvironment.GetAbsolutePath()` for: SDKReferences items, References items, Projects items, ResolvedSDKReferences ItemSpecs, RuntimeReferenceOnlySDKDependencies, ProjectName-derived paths
- [ ] Use `TaskEnvironment.GetEnvironmentVariable()` for: any registry/SDK fallbacks invoked from `ToolLocationHelper` (review)
- [ ] Use `TaskEnvironment.GetProcessStartInfo()` for: N/A

## Known complexity factors
- Several `static readonly` caches/regexes (`PlatformAliases`, `SdkReferenceFormatRegex`, `s_appxSplitChar`, `s_defaultTargetPlatformVersion`) — confirm all are immutable / thread-safe
- Calls into `ToolLocationHelper` and SDK manifest parsing — those helpers may consume process env (PATH, registry) outside `TaskEnvironment`
- Windows-only main path; `NativeMethodsShared.IsWindows` early-out
- Emits many `[Output]` ItemGroups derived from ItemSpecs — preserve original-form (Sin 1)

## 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