Seam pool stores duplicate samples of the same capture

Found during the joint-selection work (PR #154): the stored pool holds five samples that are only three distinct fits — two are exact duplicates, the same file pooled on two viewings. A capture watched twice is double-counted in every pooled statistic.

The pool should recognize a returning capture (same file identity) and refresh its sample rather than append a twin.

Out of scope for #154. Filed under the owner's standing authorization (2026-08-05: "Always feel free to file issues to keep track").