docs(u5): name the verbatim-booth boundary as U3's, not a gap

Five live booths serve the author's HTML raw and the Booth owns no
header there to put a provenance line into — it reaches those pages
through six regexes injected into arbitrary markup, which is the defect
U3 exists to fix. Their index cards carry provenance like everything
else. Written down so the boundary reads as a boundary rather than as
something this unit forgot. Verified on pewpew-ui-brief.

docs/contracts/u5_booth_manifest.contract.md

@@ -243,6 +243,13 @@ Deliberately deferred or never. Divergence here is not drift.
   manifest is a dotfile and every existing listing already skips it.
 - **Auth, or treating a manifest as trusted.** Standing non-goal; the Booth is
   LAN-internal and a hand-written `.booth.json` is a supported input.
+- **Provenance ON a verbatim-`index.html` booth's own page.** Five live booths
+  serve the author's HTML raw, and the Booth owns no header there to put a line
+  into — it currently reaches those pages through six regexes injected into
+  arbitrary markup, which is precisely the defect U3 exists to fix. Their INDEX
+  cards carry provenance like everything else; the page itself waits for U3's
+  declared embed seam. Verified on `pewpew-ui-brief`: page renders 200, card
+  reads `unannounced`.
 
 ## Invariants