[Core] Structured diagnostics surfaced to CI

[jeffpatton1971]

Summary
Introduce IDiagnosticSink and emit structured warnings/errors:

• XML2DOC001 unresolved cref
• XML2DOC002 duplicate anchors
• XML2DOC003 malformed XML
• XML2DOC004 missing
  .

Tasks

• Add diagnostics types + sink.
• Emit at relevant points in rendering.
• Unit tests for emission; integration mapping in CLI/MSBuild (separate issues).

Acceptance

• Diagnostics visible in CLI/MSBuild; builds fail only on errors.