Promote the tape=0 + from-space-scan parse-then-churn check to a CI gate for the layout-state family #7647

PERRY_JSON_TAPE=0 + PERRY_GC_FROMSPACE_SCAN=1 over a parse-then-churn workload is now a known-good end-to-end detector for the whole layout-state family. #7643 measured it: with the JSON materialiser's finalize sabotaged to (ptr, /* saw_pointer */ false) it reports dangling=8000 owners=4000 and the binary SIGBUSes; clean it reports dangling=0 and exits 0.

Nothing runs it in CI. #7643's four unit tests cover the enumerator and the relocation invariant workload-free, which is the right primary guard — but they cannot catch a new materialiser path that forgets to finalize at all, because such a path would simply not be in the test.

What the gate must assert — both halves:

1. dangling=0 in the from-space scan output, and
2. that a copying minor actually ran (copied_objects > 0).

Half 2 is not optional. This repo has shipped three gates that were green while their subject never executed (PERRY_GC_FORCE_EVACUATE inert for every gc()-driven test, #6942/#6946; the matrix's --pressure knob disabling the path it measured, #7024; a moved= counter summing two collectors, #7025). A parse-then-churn workload that happens not to trigger a copying minor would report dangling=0 and mean nothing.

Also gate PERRY_JSON_TAPE=0 itself. The whole point of #7635's dead end was that the default lazy path materialises records after the last collection. A future default change that routes this workload lazily again would silently return the gate to vacuity — so the gate should assert the parse it is measuring was eager (record count materialised before the churn, or an equivalent observable).

Not required-on-merge until it has one observed green run on main — a new gate has never been green, so promoting it immediately blocks every open PR (CLAUDE.md, "four ways a gate can be unable to fail", corollary).

Context: #7635, #7633, #7643.