fix: support auto-build with parallel tests

[ElMassimo]

GitHub Issue #376

Description 📖

This pull request makes autoBuild reliable when tests run in parallel processes.

Test workers can continue building assets on demand without requiring a separate precompilation step or runner-specific configuration.

Background 📜

Vite Ruby previously used a Mutex to serialize automatic builds. A mutex only coordinates threads within one Ruby process, while parallel test workers each have their own mutex.

When multiple workers requested stale assets together, they could launch concurrent Vite builds against the same output directory. A worker could then parse a manifest while another build was rewriting it, resulting in intermittent JSON::ParserError and ViteRuby::MissingEntrypointError failures.

This did not affect single-process test runs, and the existing mutex protected threaded requests within one process.

The Fix 🔨

Coordinate build checks and manifest reads through a project-scoped advisory file lock.

An exclusive lock covers the freshness check, Vite invocation, and metadata write. Waiting workers re-check freshness after acquiring the lock and reuse the first worker's build.

A shared lock prevents manifest reads from overlapping an automatic build. When autoBuild is disabled, manifest loading retains its previous behavior and does not initialize the builder or build lock.