ci: enforce 75% Jest coverage thresholds

[abubakarlekson27577-wq]

Title: ci: enforce 75% Jest coverage thresholds

Description:

Adds coverage enforcement to lock the quality floor.

Changes

• Enabled collectCoverage: true in the Jest config so coverage is always gathered on npm test
• Added coverageThreshold requiring ≥ 75% on lines, functions, branches, and statements
  globally

How CI is affected

No workflow changes needed — the existing npm test --if-present step in the build job already
fails the run when Jest exits non-zero, which it will do whenever any threshold is breached.
closes #108

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