[Core] Auto-link types/members in free text (IAutoLinker)

[jeffpatton1971]

Summary
Optional post-processor that links identifiers in summaries/remarks using the symbol index, while skipping code blocks and inline code.

Tasks

• IAutoLinker interface + SimpleAutoLinker default impl.
• Integrate into text rendering pipeline (opt-in via options).
• Tests: ensures no links in fenced/inline code; handles <paramref/>, <typeparamref/>.

Acceptance

• Auto-linking off by default; on-demand works and is safe.