xlstream-eval: support text comparison operators in conditional criteria

[cilladev]

Fixes: #175
Fixes: #174

Problem

Two related bugs in conditional aggregate criteria:

1. COUNTIF text operator criteria returns 0 #175: SUMIF/COUNTIF with text comparison operators (">b", ">=banana", "<banana") always returned 0. The criteria parser only handled numeric operands — non-numeric text fell through to Criteria::Equals with the full operator string as a literal, matching nothing.

2. COUNTIF/COUNTIFS operator criteria counts unique matching values, not all rows #174: COUNTIF/COUNTIFS with any operator or wildcard criteria counted matching buckets instead of matching rows. try_operator_criteria re-folded already-finished per-bucket values using AggKind::Count, which counts feeds (one per bucket) instead of summing stored row counts.

Fix

1. Text comparison variants: Add GreaterText, GreaterOrEqText, LessText, LessOrEqText to the Criteria enum. When an operator operand fails parse::<f64>(), the parser creates the text variant (lowercased) instead of falling through to Equals. Matching uses case-insensitive alphabetical comparison via a new text_for_compare helper, applied only to Value::Text cells.

2. COUNT re-fold 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 already the sum of 1.0-per-row feeds). This makes COUNTIF sum matching buckets' row counts instead of counting the number of matching buckets.

Changes

• crates/xlstream-eval/src/criteria.rs: add 4 text-comparison enum variants; update parse() fallbacks; add text_for_compare helper; add match arms; update rustdoc.
• crates/xlstream-eval/src/prelude.rs: try_operator_criteria uses AggKind::Sum finish for Count kind.
• 7 unit tests: parse, greater/greater-or-eq/less/less-or-eq text matching, case insensitivity, non-text exclusion.
• Conformance fixture with 13 Excel-populated cells: SUMIF text comparisons (5), COUNTIF text comparisons (4), COUNTIF wildcard (2), COUNTIF numeric operator (2).
• docs/architecture/aggregates.md: sync Criteria enum listing with new variants.

Test plan

• Regression unit tests added and pass (7 new, 33 total criteria tests)
• Conformance fixture created (Excel-populated) and passes
• cargo test -p xlstream-eval passes (2292 lib, no regressions)
• make check passes

Closes #175
Closes #174