~anthropic/claude-sonnet-latest → B (6:4)

Side B fixes an actual off-by-one/indexing bug (thread_post_index semantics) with real code, updated docs, and new regression tests, delivering concrete lasting value. Side A is purely a planning markdown file plus an unused RouteContext wrapper that duplicates ThreadNav without migrating any call sites, i.e., speculative scaffolding rather than executed value.