openai/gpt-chat-latest → B (4:1)

Side B introduces lasting abstractions by centralizing thread post index lookup into `ReducerState::{try_thread_post_index_chronological, thread_post_index_chronological}`, replacing duplicated indexing logic across RPC and HTML code, and adds regression tests ensuring redacted posts preserve stable chronological indices. Side A is dominated by a large project reshuffle (moving files into `legacy/`, renames, workspace/bootstrap, generated `Cargo.lock`, CI, and scaffolding) with relatively little new enduring behavior compared to the amount of churn.