Add boundary tests so the suite catches comparison mutations #192

Priority: P1

Context

Flipping > to >= in toBeGreaterThan (packages/core/src/lib/NumberAssertion.ts:423) left all 399 core tests green. Agents use this suite to decide when a change is done, so it has to catch this kind of mistake.

Do

• Add an equal-value case to every comparison-style matcher. Also cover the inclusive and exclusive edges of toBeBetween and toBeCloseTo. In scope: NumberAssertion, DateAssertion (before/after/same), and the length matchers in ArrayAssertion and StringAssertion.
• Run StrykerJS once locally on packages/core (npx -y @stryker-mutator/core with the mocha runner). Don't add it to package.json.

Done when

• The >→>= mutation and its siblings (<, <=, >=) are killed.
• The PR description lists the surviving mutants for follow-up.

Source: code audit of stackbuilders/assertive-ts at e81a614 (2026-10-13) plus open Vanta/Dependabot findings.