Normalize DESCRIPTION package name for S4 digest parity with load_all()

[Copilot]

load_fast() produced different digest::digest() values for S4 objects than pkgload::load_all(). The mismatch came from loader-created S4 metadata: the package name read from DESCRIPTION retained the read.dcf() column name, which changed the class package attribute shape and therefore the object hash.

• Root cause

  • Strip the name from the Package field when deriving pkg_name from DESCRIPTION.
  • This makes attr(class(obj), "package") match load_all() semantics for S4 instances.

• Regression coverage

  • Add a subprocess-based regression test that loads the same package with both load_fast() and pkgload::load_all().
  • Assert both loaders produce:
    • the same S4 object digest
    • an unnamed class(..., "package") attribute

• Test helper cleanup

  • Factor the digest reproduction script lines into shared variables to keep the new regression focused and easy to scan.

obj <- animal("Rex", "dog", 4)
digest::digest(obj)
attr(class(obj), "package")

Under this change, both loaders yield the same digest for the same S4 object, and the package attribute is normalized to the same shape.