fix: Refuse to make an invalid SHAMap or Ledger immutable

SHAMap::setImmutable() now returns [[nodiscard]] bool and refuses a map
already proven impossible. Every state change goes through trySetState(),
the only writer of state_ past construction, so the order between the
states is stated once: its compare-exchange can't leave Invalid however it
interleaves with another thread's, and setInvalid() stores through the same
funnel rather than behind its back. Invalid is stored unconditionally
there, since only the map itself reaches that verdict and a walk that
reaches it has to win against a thread settling the map; refusing to
overwrite Immutable would leave a map proven impossible reporting itself
sound, which is what nothing downstream could recover from.

Ledger::setImmutable()/setAccepted() do the same one level up, checking
mapsValid() before touching anything and settling both maps independently
so neither is left mid-sync because the other refused. The map hashes are
read before the maps are settled, since getHash() can unshare a dirty
tree, but written to the header only once both maps have made it. A walk
that invalidates a map in between therefore leaves the header describing
what the ledger was built from rather than a map that has since been
abandoned.

Every call site now branches on the result. The two genesis paths and
buildLedgerImpl() call logicError(), since consensus can't tolerate an
invalid ledger; the load paths return early instead; InboundLedger and
TransactionAcquire withdraw complete_ alongside the failure, since for
them a refusal is an outcome a peer can produce. A test helper that cannot
reach BEAST_EXPECT throws instead, so a refusal cannot hand a broken
ledger to the assertions below.

src/test/app/RCLValidations_test.cpp

@@ -100,7 +100,7 @@ class RCLValidations_test : public beast::unit_test::Suite
             BEAST_EXPECT(next->read(keylet::feeSettings()));
             if (forceHash)
             {
-                next->setImmutable();
+                BEAST_EXPECT(next->setImmutable());
                 forceHash = false;
             }