fix(menhir): preserve compiler stdlib in inference sandbox

[Alizter]

Description

Declare the configured compiler standard-library directory as a hidden
dependency of the ocamlc -i invocation used for Menhir type inference. This
materializes the standard library at its executable-relative location when a
package-managed compiler is invoked through a sandbox symlink on macOS, without
adding it to the compiler load path. As a result, -nostdlib and replacement
standard libraries remain respected.

Add a macOS-only regression test with a native compiler wrapper that reproduces
the platform-specific executable-path behavior.

Related Issue and Motivation

Fixes #15642.

Testing

• The macOS regression-test commit fails with Unbound module Stdlib before the
  fix.
• The targeted cram test was invoked locally and skipped on the non-macOS host.
• CI is running against the updated revision.