Pooled mean: opposing infinite tails raise a bare math.fsum error instead of a named refusal #71

_mean_with_tails in pooling.py (~lines 361-364 on main) sums tail contributions with math.fsum(s for _, s, _ in tails). When the tails hold opposing infinite sums (+inf and -inf), math.fsum raises a bare ValueError/OverflowError from the standard library, with no indication of which group, channel or tail caused it.

Expected: detect non-finite tail sums before summing and raise a named, explained refusal (consistent with the package's other refusals), so the failure is never mistaken for a data bug elsewhere. Regression test: pooling with opposing infinite tails raises the named error.

Related but distinct: #33 (tail second-moment overflow with finite true spread).

Source: ChatGPT batch 8 review, FILE item (not a release blocker).