[Tests] Matrix snapshots + anchor parity checks

[jeffpatton1971]

Summary
Harden tests to prevent regressions: matrix of type/member × per-type/single-file, parity for all AnchorAlgorithm options, auto-linker safety, and whitespace/line ending normalization intact.

Tasks

• Expand RenderSnapshots to cover link routing matrix.
• Add anchor parity tests per algorithm.
• Negative tests for auto-linker in code fences.
• Keep normalization helpers unchanged.

Acceptance

• New tests pass; snapshots stable cross-platform.