NuGet browser: finish post-install sidecar workspace reload + prerelease/restore polish

Tracking the remaining items from the now-deleted docs/plans/NUGET-BROWSER-PLAN.md (the NuGet browser feature itself has shipped; its design lives in docs/specs/NUGET-BROWSER-SPEC.md).

Functional (not yet done):

• On a successful sharplsp/nuget/install, notify the sidecar to reload the workspace (the project file changed).
• On a successful sharplsp/nuget/uninstall, notify the sidecar to reload the workspace.
• Prerelease version toggle in the browser UI.
• NuGet restore command + auto-restore on project load.

Deferred polish (explicitly low-priority in the old plan):

• Cache sharplsp/nuget/* results keyed by workspace root + dir mtimes.
• $/cancelRequest plumbing to kill in-flight dotnet restore children.
• LSP-process install-latency benchmark (<150ms) in a perf suite.
• Notification-ordering e2e (needs server→client notification capture in the test client).

The CSDEVKIT parity plan still tracks the user-facing NuGet items (prerelease toggle, restore) under "P2.4 / NuGet Package Management".

Start with docs/specs/NUGET-BROWSER-SPEC.md and the sharplsp/nuget/install and sharplsp/nuget/uninstall entry points. Use the CSDEVKIT parity plan to review the prerelease toggle and restore requirements, then inspect the existing sidecar, caching, cancellation, performance, and notification tests if present. Done means the functional and explicitly deferred checklist items are implemented and verified.