Closure dedup is case-insensitive on case-sensitive filesystems, silently dropping includes

DocumentClosure.ExpansionState.Visited is built with StringComparer.OrdinalIgnoreCase:

https://github.com/Nimblesite/SharpLsp/blob/main/sidecars/SharpLsp.Sidecar.CSharp/Workspace/DocumentClosure.cs#L192

That is correct on Windows and on case-insensitive macOS volumes, but wrong on Linux and case-sensitive APFS/HFS+ volumes, where Helpers/Foo.cs and Helpers/foo.cs are two distinct files.

Failure

A file-based app that includes both:

#:include Helpers/Foo.cs
#:include Helpers/foo.cs

visits the first, then Visited.Add(full) returns false for the second because the comparer folds case. VisitAsync returns early via RecordBound, which only records an issue when Files.Count >= MaxFiles or depth > MaxDepth — neither holds here. So the second file is dropped from the closure with no diagnostic and no logged issue. Every symbol it declares then reports CS0246/CS0103 in the editor with nothing explaining why.

The same fold applies to the cycle guard, so a genuine two-file include graph differing only in case is misreported as a cycle.

Fix

Match the comparer to the filesystem rather than the OS. OperatingSystem.IsWindows() is the wrong axis — macOS is case-insensitive by default but can be case-sensitive per volume. Probing the containing directory once per closure expansion and caching the result is the accurate approach; keying on OperatingSystem.IsWindows() || OperatingSystem.IsMacOS() is the cheap approximation.

Whichever is chosen, RecordBound should also record an issue for the already-visited case so a dropped file is never silent.

Test

Coarse e2e in WorkspaceManagerSingleFileTests.cs, guarded so it asserts real behaviour on case-sensitive filesystems only: write Foo.cs and foo.cs declaring different types, include both, assert both types bind (no CS0246).

Found while reviewing #189. Not a regression from it — the comparer predates the closure rewrite in spirit and ships with it. Tracked separately to keep #189's scope honest.

Refs [SCRIPT-CLOSURE].

Start in sidecars/SharpLsp.Sidecar.CSharp/Workspace/DocumentClosure.cs around ExpansionState.Visited and RecordBound, then inspect WorkspaceManagerSingleFileTests.cs for the coarse end-to-end coverage. Reproduce the case-sensitive filesystem scenario with Foo.cs and foo.cs, and consider the proposed filesystem probing or approximation; done means both types bind without CS0246 and a dropped already-visited file is not silent.