xlstream-eval: fix COUNTIF operator re-fold to sum row counts, not count buckets

[cilladev]

Fixes: #174

Problem

COUNTIF/COUNTIFS with operator or wildcard criteria counted matching buckets (unique values) instead of matching rows. The try_operator_criteria function re-folded already-finished per-bucket values using AggKind::Count, which counts feeds (one per bucket) instead of summing stored row counts. Example: COUNTIF(A:A,"*") with 4 text cells across 3 unique values returned 3 instead of 4.

This is the #174 re-aggregation bug. It was intended to be included in PR #188 but the prelude.rs change was lost when the workspace was deleted and re-cloned.

Fix

try_operator_criteria now finishes with AggKind::Sum for Count kind — matching how the prelude builder stores row counts (each bucket's value is the sum of 1.0-per-row feeds, finished with AggKind::Sum). The finish_kind match now explicitly mirrors prelude_plan.rs line 1195 for all aggregate kinds.

Changes

• crates/xlstream-eval/src/prelude.rs: try_operator_criteria remaps Count → Sum for finish_kind, with explicit Average → Average arm mirroring the builder.
• Conformance fixture issue-161-wildcard-blank.xlsx: 100 data rows + header, 15 Excel-populated formula cells covering wildcards (*, a*, *a*, ???-??????), text comparisons (>b, <banana, >=cherry, <=apple), exact match, and non-blank (<>). Verified against Excel on all 15 cells.

Known limitation (not this PR)

AVERAGEIF with operator criteria computes average-of-per-bucket-averages, which is mathematically wrong (should be weighted by count). Both the prelude builder and the re-fold finish with Average — the coupling is now explicit in the match. Fixing this requires storing (sum, count) pairs per bucket, which is architectural.

Scope note

COUNTIF(A:A,"") (blank count for whole columns) returns 30 (scanned rows) vs Excel's 1,048,505 (all 1M+ rows). This is a separate issue — the streaming model doesn't scan unoccupied rows. Excluded from the fixture. The wildcard-blank fix itself (wildcards matching only Value::Text) was PR #185.

Test plan

• Conformance fixture created (Excel-populated) and passes (15/15, 0 mismatches)
• cargo test -p xlstream-eval passes (2291 lib, no regressions)
• make check passes

Closes #174