[Compiler] Resolve cross-file related-location URIs in LSP relatedInformation

[cssbruno]

Follow-up from #297 and PR #300.

Problem

#297 maps diagnostic related locations to LSP relatedInformation, but the
mapping uses the current document's URI for every related location
(relatedInformationFromLang in internal/lsp/diagnostics.go). That is correct
for the single-document check the server runs today, where conflicts are
in-file, but a related location whose File differs from the open document
points at the wrong URI.

Relevant code:

• internal/lsp/diagnostics.go: relatedInformationFromLang (uses one uri)
• internal/lsp/notifications.go: publishDiagnostics (single-document check)
• internal/lang/diagnostic.go: RelatedLocation.File

Scope

Resolve cross-file related locations to their own file:// URI and compute the
range against the related file's content (loading it when the related File
differs from the open document), so a conflict that spans two files links each
location to the right file.

Acceptance Criteria

• A related location with a different File is emitted with that file's URI and
  a range computed against that file.
• Same-file related locations keep current behavior.
• A test covers a cross-file related location URI.

Verification

go test ./internal/lsp

[cssbruno]

Closing as a false positive. The LSP only ever publishes diagnostics from CheckSource on a single document (internal/lsp/notifications.go publishDiagnostics), which builds a single-page program. duplicate_route needs >=2 pages and cannot fire; route_method_conflict can only involve registrations from that one page, so every related location is already in the open document. relatedInformationFromLang using doc.URI is correct for every reachable case. This would only matter if the LSP gained workspace-wide checking, which is out of scope. Verified in PR #300.