fix(u6): fold the cold code-review panel — four-arm convergence on three surface clauses

/heid-code-review panel 01M35CK8YKEKMV7T15JXEF6A8N, verdict NOT drift-zero.
Three findings arrived from all four arms independently, and they share a
shape: a contract clause written as prose and never converted into an
assertion. That is the lens working.

- The panel dropped the added date the contract promised to show.
- `bench ls` printed no ids, and the URL it printed was truncated to 52 columns
  so the line was not pasteable into `bench state|rm`. The test's docstring
  claimed it printed ids and asserted nothing of the kind.
- `bench import` printed the description instead of the raw URL beside each
  normalized id, hiding the collapse the clause exists to expose.
- An IPv6 literal lost its brackets: http://[::1]:8080/a normalized to
  http://::1:8080/a, a broken identity that no re-post can match. Bracketed
  literals are re-wrapped; an unbracketed one is refused rather than guessed.
- A deeply-nested JSON RecursionError escaped read_benches' except pair. The
  byte cap does not help -- 200k open brackets is 200 KB.
- An empty board hid the whole benches panel, registration form included.
- The link refusal classified by captured-text emptiness, which bash can erase;
  it now answers with a B:/N sentinel so no name reads as "not a booth".

INV-4's tie-break falsifier could not fail: _write_all serializes with
sort_keys=True, so both insertion orders came back already id-sorted and
removing the tie-break left the test green. It now calls order_benches
directly. Same class as the five vacuous U4 falsifiers, found by a cold reader
rather than by us.

Also from the arms' per-invariant vacuity pass: INV-6 had no vector pinning a
non-default port as part of the identity; INV-3 asserted only that links/ was
absent; INV-8's hashed sequence omitted a read verb; INV-9's AST walk is
defeated by a string import. All closed.

Contract amended where the code was right: `updated` means last mutation, the
id cap is write-only because the id is the locator controls post back, INV-8's
file list includes the lock sidecar it always mandated. Every line number is
out of the prose -- the panel found two already stale.

565 -> 593 tests. Nothing declined.

booth/benches.py

@@ -136,6 +136,14 @@ def normalize_bench_url(url: str) -> str:
     if not host:
         raise ValueError("that URL has no host")
 
+    # RE-WRAP A BRACKETED IPv6 LITERAL. `urlsplit().hostname` strips the
+    # brackets, and rebuilding the netloc from it produces `http://::1:8080/a`
+    # — not a different spelling of the same URL but a BROKEN one, so a re-post
+    # never matches the row the operator thinks they are updating. The bracket
+    # is part of the authority's syntax, not decoration. Detected by the colon,
+    # which cannot appear in a hostname or an IPv4 literal.
+    if ":" in host:
+        host = f"[{host}]"
     default = {"http": 80, "https": 443}[scheme]
     netloc = host if port in (None, default) else f"{host}:{port}"
     # A bare "/" is the same resource as no path at all; a trailing slash on a
@@ -240,6 +248,12 @@ def read_benches(root: Path) -> tuple[list[Bench], str | None]:
         return [], str(exc)
     except OSError as exc:
         return [], f"registry could not be read: {exc}"
+    except RecursionError:
+        # Deeply nested JSON (`[[[[...`) blows the stack inside json.loads, and
+        # RecursionError is neither ValueError nor OSError — so it escaped the
+        # pair above and 500'd the page this function exists to protect. The
+        # byte cap does not help: 200k open brackets is 200 KB.
+        return [], "registry is nested too deeply to parse"
 
 
 def _write_all(root: Path, benches: dict[str, Bench]) -> None: