Close remaining rewatch build.lock gaps

[cknitt]

Motivation

This is a follow-up to the earlier build.lock fixes. Those changes covered the main rescript build path, but rescript watch still had artifact-mutating paths that could run without holding build.lock.

That matters because initialize_build is not read-only: it reads prior compile state and runs cleanup_previous_build, which can remove stale compiler assets and generated JS under lib/bs. cleanup_after_build and write_build_ninja also mutate build artifacts.

Without this follow-up, a watch process could still race with an in-flight rescript build during these remaining paths.

Newly Covered Paths

This PR adds build.lock coverage for these remaining watch cases:

• Watch startup initialization

  • watcher::start calls initialize_build to discover packages and source folders before entering the watch loop.
  • This now runs under build.lock because initialization may clean previous build artifacts.

• Watch full rebuild reinitialization

  • A full rebuild can be triggered by config changes, renames, creates, deletes, or editor atomic-save events.
  • That path calls initialize_build again to rebuild package/module state.
  • This now runs under build.lock.

• Watch full rebuild artifact writes

  • After reinitialization, the full rebuild compiles and writes build.ninja.
  • Those operations now stay inside the same build.lock boundary.

• Watch shutdown cleanup

  • When watch exits via Ctrl-C or because watch.lock is removed, it calls cleanup_after_build.
  • That cleanup now runs under build.lock.

Locking Model

• rescript build holds build.lock across initialization, compilation, cleanup, and build.ninja writes.
• watch incremental rebuilds call public incremental_build, which acquires build.lock.
• watch full rebuilds acquire build.lock before reinitialization and then call the unlocked incremental build body.
• watch startup initialization and shutdown cleanup now also acquire build.lock.
• rescript clean already acquires build.lock in the CLI entrypoint.

Callers that already hold build.lock use incremental_build_without_lock, so this avoids duplicate locking and self-deadlocks.

[cknitt]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Bravo.

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commit: 1328836