FolderAdapter(writable=True) declares WRITE+PER_PAGE, implements write() and
current_rev() (mtime token for drift detection). Conformance gains a
content-preserving positive write probe for WRITE-claiming adapters. 5 tests
green, full suite green, pyflakes clean.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
