xlstream-eval: fix COUNTIF/SUMIF with "=value" criteria returning 0

[cilladev]

Fixes: #141

Problem

COUNTIF(A2:A6,"=yes") returns 0 instead of 3. Excel supports "=value" as criteria syntax meaning "equals value". Affects SUMIF, AVERAGEIF, COUNTIFS, SUMIFS, AVERAGEIFS, MINIFS, MAXIFS.

Fix

Two root causes:

1. extract_static_criteria passes the raw "=yes" string as the composite key, but the prelude stores cell values without the = prefix (e.g. "yes"), so the direct hash lookup misses.
2. try_operator_criteria early-returns None when all criteria are Equals, preventing the fallback scan from matching.

Changes

• multi_conditional.rs: strip leading = in extract_static_criteria so composite keys match prelude entries
• prelude.rs: remove the all-Equals guard from try_operator_criteria (now always returns Value, not Option<Value>)
• prelude.rs: simplify get_multi_conditional call site (no Option unwrap needed)

Test plan

• Conformance fixture with 8 formulas: COUNTIF, SUMIF, AVERAGEIF, COUNTIFS, SUMIFS with "=value", plain "value", numeric "=10", and mixed criteria
• cargo test -p xlstream-eval --test conformance passes (85 tests, 0 failures)
• cargo test -p xlstream-eval passes (no regressions)
• make check passes

Closes #141