[Core] Pluggable anchor algorithms (IAnchorGenerator)

[jeffpatton1971]

Summary
Add IAnchorGenerator and AnchorAlgorithm { GitHub, Docsify, Kramdown }. Use it for both emitted anchors and link targets.

Tasks

• Implement IAnchorGenerator and GitHubAnchorGenerator (default = current).
• Wire into member/type anchor emission + link construction.
• Option: RendererOptions.AnchorAlgorithm.

Acceptance

• Default output is byte-for-byte identical to 1.3.1.
• Parity tests ensure emitted anchors == link targets per algorithm.