feat(manifest): U5 — booths that say who posted them and why

The index card showed a name, an item count and a countdown, and nothing
the poster chose. An agent with something to show therefore had no way to
make the booth say "look at this" and posted a URL to the link board
instead — which is why 145 of that board's 210 rows (69%) ended up
pointing at booths that had already been swept. The board was absorbing a
job it was never shaped for. This is the shape.

Each booth carries `.booth.json` — {handle, title, why, created} — written
by the CLI from $ALTHING_HANDLE, and the provenance line renders on both
index lanes and on the booth page header.

WHAT IS WHERE

- booth/manifest.py, stdlib-only and importing nothing from booth.* either:
  scripts/booth imports it under the system python3 with no venv, and a
  cross-import between two stdlib-only modules is a second way for that
  invariant to break. It joins the shared test_stdlib_only list and keeps
  a stricter copy of its own.
- The read is lenient and cannot raise. list_booths touches every booth on
  every index load, so a manifest that cannot be parsed costs that booth's
  provenance and nothing else. That is the v0.2.2 lesson applied before the
  same mistake rather than after it.
- Absent and damaged render differently — `unannounced` and `unreadable`.
  Folding "cannot be read" into "never said" would hide the one case
  somebody has to go and fix.
- Re-announcing preserves `created`. A second `booth add` sharpening the
  why is not a second appearance of the booth.
- The write is atomic (invariant 5); the temp file is itself a dotfile, so
  no listing can see it mid-write.

THREE OPERATOR CALLS, 2026-09-22

Flags on the existing new/add verbs rather than a separate `announce` verb
(a second step is the step that gets forgotten, which is the rot's own
mechanism). Unannounced booths get a quiet marker rather than nothing — the
convention is only adoptable if the gap is visible. U5 adds provenance only
and does NOT add a second index ordering keyed on announcement time; that
is a different surface needing its own stated rule, parked for v1.1.

NO EXEMPTION LIST

A pickup booth and the standing link board are created by the service, so
they announce themselves with handle `booth`, which is true rather than
manufactured. One rule — a booth with no manifest is unannounced — instead
of a growing set of special cases.

ALSO

tests/test_booth.py's keep/release assertion was slicing the page on the
bare word `boothhead`, which has lived in the stylesheet far longer than
the assertion has; it was reading CSS and passing on luck, and went red the
first time a new rule landed above the old one. Same assertion, aimed at
the markup. A U5 test had the mirror-image bug: pytest derives tmp_path
from the test name and the index renders data_dir, so a test named
`test_an_unannounced_booth_says_so` put the needle in the haystack itself
and passed against a template that did not yet exist.

310 tests (304 before this unit's CLI half). Live service restarted, 26/26
booth pages verified 200, end-to-end smoke through the real CLI.

NOT TAGGED. The cold contract-review panel is still in flight and the
code-review and bug-hunt gates have not run. Tagging with a gate
outstanding is what made v0.2.0 premature.

persistent-memory.md

@@ -26,7 +26,18 @@ _As of 2026-09-22:_
   the deploy. U1 `ce598b3`; U2 `c7f9437` released as `v0.2.0`, then `5e41108` as
   `v0.2.1` (four contract-panel findings), then `v0.2.2` carrying the
   **bug-hunt panel's** nine (below).
-- **U5 is the next unit** (operator, 2026-09-21): **self-announcing booths.**
+- **U5 is IMPLEMENTED and unreleased** as of 2026-09-22. `booth/manifest.py`
+  (stdlib-only, INV-1), `.booth.json` per booth, the provenance line on both
+  index lanes and the booth page header, `--why` / `--title` on `booth new` and
+  `booth add`, and the link board + pickup booths announcing themselves as the
+  service's own. 310 tests, live service restarted, 26/26 booth pages verified
+  200 and all 26 rendering `unannounced`. **Deliberately NOT tagged yet**: the
+  cold `/heid-contract-review` panel is still in flight and the code-review and
+  bug-hunt gates have not run. That ordering is the 2026-09-21 lesson applied —
+  a release whose gate is outstanding is premature even when the tier is right.
+  Contract: `docs/contracts/u5_booth_manifest.contract.md` (carries its own
+  seam-review section).
+- **U5's original framing** (operator, 2026-09-21): **self-announcing booths.**
   `.booth.json` carrying `{handle, title, why, created}`, written by the CLI from
   `$ALTHING_HANDLE`; the index card gains provenance and a one-line purpose, and
   the index becomes the "what landed" feed the link board was being used as. It