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fix(list): fix scan_left evaluating callback twice per element

scan_left was calling f(acc, x) twice per iteration - once to store
the result in the new list node and again to compute the next
accumulator. This is a correctness bug for effectful or
non-deterministic callbacks, and doubles work for pure callbacks.

Fix: compute f(acc, x) once and reuse the result for both purposes.

Also fix the empty-list minimum() test to actually assert None
instead of ignoring the result (matching the maximum() test pattern).

Add regression test that verifies f is called exactly once per element.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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