fix: stabilize public Vitest baseline

[Rajioba1]

Summary

• allow empty datasets to complete spreadsheet activation while keeping stricter mutation readiness checks
• repair stale frontend fixtures, service doubles, and formula-display lifecycle coverage
• cap Vitest workers adaptively, collect private helper contracts only when their complete local dependency closure exists, and enforce Vitest in CI

Validation

• Vitest: 248 files, 3,407 passed, 0 failed, 1 pre-existing explicit unit skip, 0 unhandled errors
• TypeScript typecheck: passed
• production build: passed
• license summary and isolation allowlist checks: passed
• cargo check: passed
• independent whole-branch review: approved with no findings

Scope notes

• no documentation or private E2E files are included
• the full Rust test suite has existing macOS-only failures (Accessibility permission, a Windows-path assertion, and parallel cache-test interference); representative failures reproduce on the untouched upstream commit and this branch changes no Rust files