Enforce a coverage floor #195

Priority: P2

Context

Coverage is high today (branches: core 96%, dom 93%, sinon 93%, native 76%), but nothing stops it from dropping. An agent can add an untested matcher and CI stays green.

Do

• Add c8 as a root dev dependency and a coverage script to each package.
• Set thresholds to lines 95% and branches 90%. Start native branches at 75%.
• Add coverage/ to .gitignore.
• Wire coverage into each package's check script so CI enforces it through yarn check (see Make CI run the real checks and a dependency audit #193).

Done when

Removing a test from a well-covered file drops coverage below the threshold and fails yarn check.

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