PR Summary

Low Risk
Low risk: this is limited to test/CI configuration and devDependencies, with no production runtime behavior changes. Main risk is CI failures if coverage thresholds or include/exclude patterns don’t match current test suite.

Overview
Adds code coverage reporting for Vitest using @vitest/coverage-istanbul, including a new test:coverage script and coverage configuration (reporters, include/exclude, and 80% line/function thresholds) in vitest.config.ts.

Updates CI (.github/workflows/test.yml) to run test:coverage instead of test, and ignores generated coverage/ artifacts via .gitignore; lockfile is updated accordingly.

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