~anthropic/claude-sonnet-latest → A (3:2)

Commit A fixes a real bug (unranked siblings being merged into a single nav group instead of one group per isolate) and adds a targeted regression test verifying the corrected grouping. Commit B is a large structural refactor splitting forum.rs into submodules plus an unrelated new sampling script, which is useful for maintainability but introduces no behavioral improvement and carries higher risk of noise/regressions without new tests.