docs(plans): mark PR-4 of error-variants audit as shipped (#587) (#588)

Expand the PR-4 section with as-shipped detail: pub(crate)
visibility on the new constructors, post-#586 line numbers,
the test-fixture no-op (cursor wrapper auto-promotes), the
/simplify dedup of the parse_unix_seconds_str closure, and
three deferred follow-ups (collapse parse_field+parse_proto,
Into<String> → Display, shared parse_unix_seconds helper).