Engine.history, usage, activatedTools, modelCaps, turnOpts, and
cfg.Provider/Model are now mutated and read under e.mu. The lock is
released across blocking provider.Stream calls so external setters
(SetProvider, SetHistory, InjectMessage, etc.) can interleave.
History() now returns a copy. Snapshot helpers (latestUserPrompt,
historySnapshot, snapshotTurnOpts, etc.) replace the unsynchronised
reads scattered through runLoop and buildRequest.
Closes audit finding H4. Adds a race regression test that fails under
-race before the fix and passes after.
vikingowl
5cd3ccd931