Publish optional zero-reference diagnostics

[Ky6uk]

Summary

Publish zero-reference findings through an optional VS Code diagnostic collection.

Motivation

Diagnostics provide native Problems navigation and can mark unused declarations without introducing a custom view. DiagnosticSeverity.Hint together with DiagnosticTag.Unnecessary matches the meaning of a conservative zero-reference finding.

Scope

• Create a named Zero Reference diagnostic collection.
• Add an opt-in diagnostics setting; keep existing CodeLens behavior unchanged by default.
• Publish diagnostics at the exact declaration-name range.
• Use DiagnosticSeverity.Hint, DiagnosticTag.Unnecessary, and a stable diagnostic code.
• Reuse existing analysis results instead of running a second reference lookup pass.
• Replace or clear stale diagnostics when a document is reanalyzed, disabled, removed, or no longer supported.
• Preserve raw provider failures in a Zero Reference output channel without showing technical notifications to users.

Acceptance criteria

• Findings appear in the Problems panel and navigate to the declaration.
• Unnecessary declarations use the editor theme's native unnecessary-code presentation.
• Disabled diagnostics leave no Zero Reference entries behind.
• Used, ambiguous, external, and unresolved symbols do not produce diagnostics.
• Diagnostic lifecycle and error logging have focused Extension Host tests.

Dependencies

• Keep analysis results fresh across files and cache unchanged documents #17 for authoritative cached analysis and cross-file invalidation.

Out of scope

• Workspace file discovery and traversal; that is tracked in Add a cancellable workspace-wide zero-reference scan #3.
• Automatic code deletion.