feat: add org-drill-on-timeout-action to drop unfinished cards at the time limit

Implements upstream #56. When org-drill-maximum-duration is reached, the session used to keep presenting the in-progress card and the again-queue until they drained, so the only way out was to finish them or interrupt. I added the defcustom org-drill-on-timeout-action with a discard-current value that ends the session as soon as the limit is hit, leaving the dropped cards untouched: they keep their existing scheduling and turn up again next session. The default, finish-current, preserves the old behavior.

The gate lives in org-drill-entries-pending-p, the single predicate the drill loop checks between cards. Under discard-current past the limit, the in-progress and again items stop counting as pending, so the loop ends instead of draining them.

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