exthost.service: the storage quota, on the machine

The reference package declares storage:1. The test writes 2 MiB into its
data directory and watches the host measure it, stop it, and show the
quota's reason, then clears the data, enables it again, and sees the
service come back. That is the operator's way out, so the test walks it.

Proven. The unit suite: 451 tests. On the bench reference, image
20260921190848 with the cross-built extension host mounted over the
image's: exthost.service PASS, and exthost.events and
exthost.hold-pause-tier PASS beside it. The first run of this check
caught a real bug: the reason arrived empty, because the host built it
in the buffer the supervisor overwrites when it stops a service.

Acceptance. exthost.service already covers forgeext's **; the quota is
part of what the host does to a confined service, so it is exercised
there.

forgetest/forgetest/suite/exthost.py

@@ -977,6 +977,31 @@ def service(ctx):
                   and (rep.get("api_traversal") or [0])[0] == 400,
                   "a hold it was not granted, a path the API does not have, a path with ..: %s %s %s",
                   rep.get("api_hold"), rep.get("api_nowhere"), rep.get("api_traversal"))
+        # the storage quota: the host measures what it holds and sets it aside
+        big = os.path.join(EXT_ROOT, "data", REF_ID, "toobig")
+        with open(big, "wb") as f:
+            f.write(b"x" * (2 << 20))
+        ctx.log("wrote 2 MiB into its data directory (it declares storage:1)")
+
+        def quarantined():
+            x = _svc(REF_ID)
+            return x if x.get("state") == "quarantined" else None
+
+        q = _until(ctx, quarantined, 90, poll=1.0)
+        ev["quota"] = {"state": (q or _svc(REF_ID)).get("state"), "reason": (q or _svc(REF_ID)).get("reason")}
+        ctx.log("over its quota: %s", ev["quota"])
+        ctx.check(q, "a service 2 MiB over its 1 MiB quota was not set aside: %s", _svc(REF_ID))
+        ctx.check("may hold" in ((q or {}).get("reason") or ""),
+                  "it was set aside without the quota's reason: %s", (q or {}).get("reason"))
+        ctx.check(not os.path.exists("/proc/%d" % pid), "it is still running after being set aside")
+        # The operator's way out: clear the data, enable it again.
+        os.remove(big)
+        r = _forgeext("enable", REF_ID)
+        ctx.check(r.get("ok") is True, "enable after the quota -> %s", r.get("error"))
+        back = _until(ctx, lambda: _svc(REF_ID) if _svc(REF_ID).get("state") == "running" else None, 90, poll=0.5)
+        ctx.check(back, "it did not come back once its data was cleared and it was enabled: %s", _svc(REF_ID))
+        pid = (back or {}).get("pid") or pid
+
         sock = "%s/%s.sock" % (API_DIR, REF_ID)
         st_ = os.stat(sock)
         ctx.check(stat.S_ISSOCK(st_.st_mode) and stat.S_IMODE(st_.st_mode) == 0o660 and (st_.st_uid, st_.st_gid) == (0, uid),