README gains an Admission section covering both routes, the admin commands,
and the six new config keys. The author's side leads with Twt-Register and
says plainly not to accept this mailbox: doing so makes a client attach an
accept token to every later mail, and a record is verified by rebuilding the
exact signed bytes, which cannot include one — so every post would be refused.
audit.md's "requests-tier mail is structurally invisible" was a stated
hardening property, and this change deliberately reverses it. Rewritten as the
chain of gates that replaces it, each named with the probe that proves it, and
the feed-path property restated: the grammar widened for host-namespaced
slugs, so traversal stopped being structurally impossible and became a rule
that has to be right. Added the subject-truncation property the rig found.
PLAN.md was the design document and is kept as the record of intent, with its
status corrected — it still claimed nothing in src/ existed — and a "since
implemented" note wherever the shipped behaviour diverges, rather than quietly
editing the original intent away.
