fix(u3): seven defects two cold panels found in the declared seam

The /heid-code-review and /heid-bug-hunt panels, artifact-only over the U3
diff, between them found four real defects and three vacuous falsifiers. Both
snapshots predate the contract-review fixes, so two of their findings were
already closed; the rest are here.

Prototype pollution in the placement maps. A mark id and a question key are
both [A-Za-z0-9][A-Za-z0-9._-]*, so `toString` and `constructor` are legal in
each. Against a plain `{}` an anchor naming NO mark returned an inherited
function, passed the guard meant to reject it, and threw on .questions.length
-- aborting placement before the tail, so one typo in author markup cost the
page every ask. The `placed` set had the mirror bug: inherited
`got.constructor` read as already-placed and silently dropped a question.
Object.create(null), three times. Found independently by both panels.

A declaring page was not served as written. read_text() opens in
universal-newline mode, so a CRLF report came back LF, and errors="replace"
replaced every byte that was not valid UTF-8. That is this unit's headline
promise, broken by the read itself, and the test could not see it because its
fixture was LF-only ASCII. The verbatim branch reads and serves bytes now; the
decoded copy answers only "does it declare the seam?".

A submit anchor inside the author's own <form> lost ours -- the parser drops a
nested form element outright -- while the code still recorded the pick as
submitted, so no fallback was appended. Every control's form= pointed at
nothing and the button did nothing. It counts as submitted only if the form
survived.

A broken pick's diagnostic never rendered from a submit-only anchor: an errored
pick's submit block is empty, and mounting that then marking it placed made the
tail skip the "broken ask" box entirely. The anchor is left alone instead.

An author's own element could hijack the open-ask chip -- id="bk-ask-winner-
background" satisfies any prefix rule, hyphen boundary included. The chip now
searches only elements this script mounted, which is the identity the deleted
bk-ask-<id>-top anchor used to guarantee, and takes the earliest by
compareDocumentPosition.

No error boundary around fragment rendering. A .marks.json that is well-formed
JSON with a wrong-shaped answer hydrates with no error and then raises in the
macro; this endpoint renders every pick on every load of the report, so that
was the whole seam gone while hold_read called the file readable. Reproduced
before building for it. _safe_fragments gives it the per-mark leniency
_hydrate_safe already applies one layer down.

The gallery and marks pages still 500 on that same entry. Measured at 42ea67f
-- it predates this unit, they render the same macro with no guard, and the
gallery is named out of scope in the contract. Recorded, not quietly widened:
persistent-memory.d/2026-09-22-a-wrong-shaped-answer-500s-the-gallery.md

Also corrected: several comments claimed a multi-question pick POSTs a 400
unless every question is answered. It does not -- an empty submission is
refused, a partial one is recorded on purpose. The real reason an unplaced
question must still be appended is that a question which never reaches the page
cannot be answered at all.

Vacuity pass rebuilt around the rule this session learned: the mutation comes
from the invariant's claim, never from the falsifier's example. 21 mutations,
21 caught, unmutated control green. Getting there took three rounds -- it
passed INV-3 with the contract's own mutation, then found its own fix's hole,
then flagged seven stale mutations and one genuinely vacuous fixture whose
sibling-mark arrangement made the right answer also the first answer.

444 tests. Deployed and verified: 23/23 booths 200, and all four live verbatim
reports served at exactly +46 bytes -- len(EMBED_SCRIPT_TAG) -- with the
authors' own wrappers and headings intact and no console errors.

persistent-memory.md

@@ -49,13 +49,15 @@ _As of 2026-09-22:_
   a JavaScript dependency it did not have, and an author-facing anchor syntax —
   and **minor needs his explicit approval**. `pyproject.toml` still says
   `0.4.0`; nothing is tagged. Do not bump it on your own.
-- ⚠ **TWO U3 GATES ARE STILL IN FLIGHT** and must be drained, triaged and
-  answered: `/heid-code-review` (`01M352RXV1ZET566KV73C7TSB8`) and
-  `/heid-bug-hunt` (`01M352TPCSN52G6NGJ07T5WSGY`), both dispatched 17:32Z.
-  **The `/heid-contract-review` panel (`01M351WKV666D681SSRNY7D7X6`) is CLOSED**
-  — 12 findings, 10 adopted, 2 already settled, 1 declined, reply sent. Two of
-  its adopted findings were CODE fixes, not wording: the vacuous INV-3 falsifier
-  and the bare-substring seam detection. The **seam review ran in-session and is
+- **ALL FOUR U3 GATES ARE CLOSED.** In-session seam review (5 findings, SR-2 a
+  real payload-shape bug); `/heid-contract-review`
+  (`01M351WKV666D681SSRNY7D7X6`, 12 findings, 10 adopted, 2 already settled by
+  the seam review while it was in flight, 1 declined);
+  `/heid-code-review` (`01M352RXV1ZET566KV73C7TSB8`, 3 more vacuous falsifiers
+  + the prototype-pollution bug); `/heid-bug-hunt`
+  (`01M352TPCSN52G6NGJ07T5WSGY`, 5 net-new, incl. the byte-exactness break).
+  **Seven of the adopted findings were CODE fixes, not wording** — the cold
+  gates were not ceremony on this unit. The **seam review ran in-session and is
   folded in** — five findings as a table at the end of the U3 contract, and SR-2
   was a real payload-shape bug the cold panel structurally could not see. U4's three and U5's three are all closed
   (`01M34VX0SH23Y3VC92E7GM4S70`, `01M34WAFJC3RTERFYBBZJN1SVG`,
@@ -86,6 +88,7 @@ _As of 2026-09-22:_
 ## Recent decisions
 
 - `[2026-09-22]` **U3 landed — the page declares the seam, the Booth mounts into it** — ten regexes against author HTML replaced by a substring test and a `+` → `persistent-memory.d/2026-09-22-u3-declared-embed-seam-landed.md`
+- `[2026-09-22]` **A wrong-shaped answer 500s the gallery and the marks page** — PRE-EXISTING (measured at `42ea67f`), NOT U3; the v0.2.2 lesson is only half-implemented → `persistent-memory.d/2026-09-22-a-wrong-shaped-answer-500s-the-gallery.md`
 - `[2026-09-22]` **The browser became a test surface** — READ BEFORE TOUCHING `playwright` IN pyproject; the pinned upper bound is the foot-gun, and these tests SKIP rather than fail → `persistent-memory.d/2026-09-22-the-browser-became-a-test-surface.md`
 - `[2026-09-22]` **A vacuity pass that tries the contract's own mutation agrees with itself** — U3 ran one, reported 7/7, and a cold panel then showed one of the seven was vacuous; READ BEFORE WRITING A *Falsifiable:* LINE → `persistent-memory.d/2026-09-22-seven-of-seven-falsifiers.md`
 - `[2026-09-22]` **U4 landed — lifetime is derived, not declared** — three states, viewing is activity, and no new arithmetic anywhere → `persistent-memory.d/2026-09-22-u4-derived-lifetime-landed.md`