[eval] Do not mix Spider EX and BIRD EX in the same score card

[cursor]

Context

ADR #40 metrics catalog: Spider EX vs BIRD EX differ (result-set comparison / value normalization / official scripts).

Proposal

• Score bundle must record ex_variant (or equivalent): spider | bird | custom.
• UI must never present a single undifferentiated "EX" across mixed benches without the variant tag.

Why

Mixing EX variants invalidates comparisons and paper claims.

Acceptance

• Field present in schema; UI shows variant next to EX when both benches appear.