Adopt Visual Studio Shared Assembly Cache

## Context

Visual Studio is introducing a Shared Assembly Cache to improve assembly loading performance. MSBuild needs to integrate with this infrastructure.

## Current Status (as of June 10, 2026)

1. **MSBuild Locator changes** — a concrete locator change is needed on the MSBuild side. This is the immediate next step and is being prepared for review.
2. **Shared Assembly Cache team prototype** — the partner team is prototyping changes using an experimental insertion from the MSBuild internal build to validate the approach.
3. **No blocking dependency** — the MSBuild team is not blocked; the locator change can proceed independently.

## Next Steps

- Complete and review the MSBuild Locator change.
- Support the partner team's prototype as needed (review PRs, help with experimental insertions).
- Once the partner prototype validates the approach, help get those changes merged.

## Notes

- Discussed in grooming on June 10, 2026.
- The locator change is described as relatively simple.