locator has the data of the extension of a .dkk, the locator sealed with
tlock for the round of the DateKey, and the envelope: the .dkc in age, split
into a header that the locator carries and a rest that can hide inside
another file. The plaintext of the locator measures the least multiple of
4096 bytes that holds it. Nothing is downloaded: a reader gives the rest to
OpenEnvelope, which checks its size and digests.
README and CHANGELOG describe what the draft adds.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
