Build History for branch/multicore-safety

Improve watch.ml comments and error message Replace "Not sure this is right" with a description of what stream_iter does and its limitation (None is ignored). Replace "Big Yikes" failwith with an actionable error message. Replace "terrible hack" doc with an explanation of why the global switch exists and what the proper fix would be. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Update irmin.mli examples from Lwt to direct-style Migrate the custom_merge and sync code examples to direct-style (remove open Lwt.Infix, >>=, >|=, Lwt_main.run, Lwt_list). Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Remove redundant | e -> raise e exception re-raises These catch-all branches in try/with blocks simply re-raise the exception and have no effect. Removing them for clarity. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Protect in-memory store cache with a mutex in irmin_mem Add a mutex around the find-or-create pattern on the global cache hashtable to prevent concurrent fibers from creating duplicate store instances. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>