embedding-similarity accepts out-of-range threshold

[royalpinto007]

Problem

threshold: NaN never passes (even identical texts), threshold: -5 always passes (even junk): the threshold is used raw with no range check.

Where

src/scorers/embedding-similarity.ts (typeof spec.threshold === "number" ? spec.threshold : 0.8, then score >= threshold).

Acceptance

Add a failing test first asserting threshold: NaN (and <0/>1) fails validation or is clamped. Then fix.