docs(hv/learner): NewLearner / Learner::new take an extra trailing arg

Go: hv.NewLearner(model, seed, nil)
Rust: Learner::new(model, seed, None)
Python: unchanged (the new third argument defaults to None).

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docs(deque): add empty check (DequeIsEmpty / deque_is_empty)

Document the direct sentinel-anchor query for testing whether a deque
has any member carriages. Includes Python free-function form.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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