feat(u3): a verbatim report declares the seam, the Booth mounts into it

A booth that ships its own index.html was served through ten regular
expressions applied to markup the Booth did not write: six in
wrap_verbatim_html hunting for somewhere to hang a favicon and a chip, four
in booth/inline.py substituting rendered ask markup into the author's own
tags. Both worked. Both were the most fragile thing in the service, on the
path the operator uses most.

The whole class is replaced by a declared seam. A report carries one line —
<script src="/_booth/embed.js" defer></script> — and the chrome mounts
through DOM APIs. What the server does to author HTML is now, in full:

    return html if declares_embed(html) else html + EMBED_SCRIPT_TAG

Two substring tests and a concatenation. Both of the old wrapper's hard
constraints stop existing rather than being satisfied more carefully:
nothing can displace a leading doctype into quirks mode and nothing can push
the charset meta out of its detection window, because nothing in front of
them ever moves. A page that declares the seam is served exactly as written.

Fragments are still rendered by the _ask_inline.html macros and handed over
GET /b/<name>/embed.json; embed.js places them and decides nothing. Openness
comes from open_marks, order from (created, id), questions in declaration
order. A single-question pick normalizes to key None, so the payload carries
questions as a list rather than an object — keying by name would serialize
that as the string "null".

Placement is an anchor fill, not a replacement: el.insertAdjacentHTML(
'beforeend'), so an author's wrapper and its contents survive. The regex it
replaces was eating the opening tag of dfa-concepts' styled .ask blocks and
orphaning their headings, live, unreported.

data-booth-mark is canonical; data-booth-ask stays a kept alias because two
live reports use it. The comment placeholders are dropped — no users.

Declared cost: the verbatim path now needs JavaScript. The never-invisible
guarantee holds through the index badge and /b/<name>/marks, both of which
render server-side.

Deleted: booth/inline.py entire, wrap_verbatim_html and its six patterns,
_BACK_CHIP, asks_chip, inject_asks, FAVICON_LINK, the styles() macro.

Tests 410 -> 434. tests/test_embed_browser.py drives a real Chromium: the
placement algorithm and the form= binding of a scattered multi-question form
cannot be observed any other way, and that binding was measured rather than
assumed (N=3 per condition, with a form-first positive control and a
points-at-nothing negative control).

Contract: docs/contracts/u3_declared_embed_seam.contract.md, with the
in-session seam review and the cold contract panel both recorded. Two of the
panel's findings were code fixes: a vacuous INV-3 falsifier that a renamed
regex walked straight through, and a bare-substring seam detection that read
a report merely quoting the path as declaring it and silently served it with
no chrome.

pyproject.toml

@@ -15,6 +15,15 @@ dependencies = [
 test = [
     "pytest>=8.0",
     "httpx>=0.27",  # fastapi TestClient
+    # U3's embed seam moves placement into the browser, where no string
+    # assertion can see it. Browsers are NOT downloaded per project: they live
+    # box-wide in /opt/ms-playwright with PLAYWRIGHT_BROWSERS_PATH wired
+    # globally. THE UPPER BOUND IS THE POINT -- each playwright release pins a
+    # Chromium revision, and one that wants a revision the shared store does
+    # not have dies with an opaque "Executable doesn't exist" rather than a
+    # missing-dependency error. 1.60-1.62 map to chromium 1223/1228/1234, all
+    # present. Raise the bound only after the store has the newer revision.
+    "playwright>=1.60,<1.63",
 ]
 
 [build-system]