Enforce test coverage thresholds in CI

[i-afaqrashid]

Summary

Add V8 coverage to the vitest run and enforce minimum thresholds so coverage can't silently regress.

Why

The packages are already well covered, but nothing guards that. A threshold gate in pnpm verify (which CI runs) keeps coverage from drifting down as the code grows.

Scope

• Add @vitest/coverage-v8 and a test:coverage script.
• Configure coverage over packages/core/src and packages/react/src, excluding test files.
• Set thresholds a few points below current coverage as a safe floor (statements/lines 95, functions 95, branches 90).
• Run coverage as part of verify so the gate applies locally and in CI.

Acceptance

• pnpm test:coverage enforces the thresholds and passes at current coverage.
• pnpm verify runs coverage and passes.

[i-afaqrashid]

Reopening. The first attempt wired @vitest/coverage-v8 into the verify chain but the dependency didn't actually land, so verify broke outside the plain test step. I reverted that in #138 to keep main green. Will redo this properly: commit the coverage dependency and lockfile together, confirm verify passes, and only merge on green CI.