feat(pool): no machine-wide width - 128 passes per available account

Jorijn: "there is no need to do a global cap anymore, the cap should be
128 per available account". `--concurrency` no longer defaults to 128:
the limits are each OAuth account's 128 (and the local login's), every
driver's counted, an API seat's own REVIEW_GATES_API_CONCURRENCY, and
free memory. `--concurrency N` still caps the machine when given; a value
below 1 is a usage error.

The dump's args.concurrency is null then. The board's driver settings
read `128 per account`, which every driver's `seats` shows as that
driver's width, and the `seats` header says a width is that driver's cap.
The Scheduler test runs 300 passes on two accounts: 256 at once, 128 each.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

implementations/Crafter.Review-Seats.cpp

@@ -1009,6 +1009,6 @@ namespace Crafter::Review::Seats {
                                         FormatWhen(NumberOr0(Dyn::Get(d, "since")), now, zone), Dyn::Str(Dyn::Has(d, "inflight") ? d.at("inflight") : zero), held));
             total += static_cast<std::int64_t>(NumberOr0(Dyn::Get(d, "inflight")));
         }
-        return std::format("drivers ({} in flight on this machine; the width is the machine's cap): {}", total, Dyn::Join(parts, "; "));
+        return std::format("drivers ({} in flight on this machine; a width is that driver's cap): {}", total, Dyn::Join(parts, "; "));
     }
 }