test: add core coverage gate

[max23468]

Summary

• add a canonical Vitest coverage command and core-module threshold config
• expand tests for storage/fs/time and critical fetch/report/summary/text branches
• add a PR quality workflow for test, core coverage and build on Node 24
• document the Atlas coverage workflow in README and toolchain

Atlas Coverage

• configured core perimeter: src/config.ts, src/dashboard.ts, src/fetch-resource.ts, src/fs.ts, src/report.ts, src/storage.ts, src/summary.ts, src/text.ts, src/time.ts, src/url.ts
• threshold: 75% lines, 65% branches
• measured baseline on the same perimeter: 71.65% lines, 53.26% branches
• measured current result: 87.4% lines, 65.35% branches

Residual Gap

• numeric gap vs Atlas threshold on the configured core perimeter: none
• stability margin on branches is still narrow: +0.35 points over the 65% threshold
• qualitative gap still visible in covered core files: src/dashboard.ts branch coverage is 43.8% and remains the next high-value target
• out of scope for this PR: src/index.ts, src/scan.ts, src/email.ts, src/dashboard-publish.ts

Next Step

• add a focused follow-up on src/dashboard.ts states and branches to raise branch margin above the bare threshold and make the core perimeter less fragile

Verification

• npm test
• npm run test:coverage
• npm run build
• git diff --check

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