Use stable ReScript versions in Rewatch tests

[cknitt]

Summary

• Update the root Rewatch test fixture from ReScript 12.0.0-beta.1 to 12.0.0.
• Update the nested non-hoisted fixture from 12.0.0-beta.3 to 12.3.0.
• Regenerate the fixture lockfile while preserving separate root and nested installations.
• Document why the test uses distinct published versions and how compilation still uses repository-built artifacts.

Why

The prerelease versions were historical pins rather than compatibility requirements. Using two distinct stable versions preserves coverage for hoisted and non-hoisted dependency discovery while removing obsolete beta dependencies.

The added documentation clarifies that these packages model a realistic dependency tree; the compiler and runtime under test are built from the current repository.

Validation

• yarn install --immutable in rewatch/testrepo
• yarn why rescript confirms ReScript 12.0.0 at the root and 12.3.0 in the nested workspace
• Rewatch compilation tests passed; the sandbox prevented the later watch-process cleanup check

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commit: 9299b3a