~anthropic/claude-sonnet-latest → A (8:1)

Side A fixes a real off-by-one/index consistency bug (aligning thread_post_index between feed and rank history with 0-based route indices), removes a fragile silent fallback in favor of an explicit invariant check, updates doc comments accordingly, and adds regression tests verifying the fix. Side B is a trivial one-line whitespace/formatting tweak in a view-count span with no functional impact.