trie/bintrie: test DeleteAccount produces a deterministic root

Two independent tries running the same UpdateAccount → DeleteAccount
sequence must produce identical root hashes. Deletion does not
return the trie to a pristine-empty root (zero blobs hash to
non-zero leaves under the tombstone model), but the post-delete
root must be deterministic across runs — any non-determinism in
the tombstone-write path would silently fork the network on the
first self-destruct after enabling flat state.

Also pin the post-delete root != empty root invariant so a future
change to the tombstone semantics surfaces here instead of in
GetAccount's deletion-detection branch.

trie/bintrie/trie_test.go

@@ -555,6 +555,48 @@ func TestDeleteAccountPreservesHeaderStorage(t *testing.T) {
 	}
 }
 
+// TestDeleteAccountHashIsDeterministic verifies that two independent tries
+// running the same UpdateAccount → DeleteAccount sequence produce identical
+// root hashes. This is the consensus-critical property of the tombstone
+// model: deletion may not produce a pristine-empty root (zero blobs hash to
+// non-zero leaves), but it MUST be deterministic across independent runs.
+//
+// A regression that introduced any non-determinism into the tombstone write
+// path (e.g. relying on map iteration order, randomized offsets) would
+// silently fork the network on the first self-destruct after enabling flat
+// state.
+func TestDeleteAccountHashIsDeterministic(t *testing.T) {
+	addr := common.HexToAddress("0x1234567890abcdef1234567890abcdef12345678")
+	codeHash := common.HexToHash("c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470")
+	acc := makeAccount(42, 1000, codeHash)
+
+	run := func() common.Hash {
+		tr := newTestTrie(t)
+		if err := tr.UpdateAccount(addr, acc, 0); err != nil {
+			t.Fatalf("UpdateAccount: %v", err)
+		}
+		if err := tr.DeleteAccount(addr); err != nil {
+			t.Fatalf("DeleteAccount: %v", err)
+		}
+		return tr.Hash()
+	}
+
+	first := run()
+	second := run()
+	if first != second {
+		t.Fatalf("non-deterministic root after Update+Delete: first=%x second=%x", first, second)
+	}
+
+	// Sanity check the tombstone model itself: the post-delete root is NOT
+	// the empty-trie root, because zero blobs hash to non-zero leaves. If
+	// this assertion ever flips, the tombstone semantics have silently
+	// changed and the deletion-detection branch in GetAccount needs review.
+	empty := newTestTrie(t).Hash()
+	if first == empty {
+		t.Fatalf("post-delete root unexpectedly equals empty-trie root %x; tombstone semantics changed", empty)
+	}
+}
+
 func TestBinaryTrieWitness(t *testing.T) {
 	tracer := trie.NewPrevalueTracer()