test: add FuzzMarshal, golden messages, synctest cancellation and cross smoke

Assisted-by: GLM 5.3 Flash

justfile

@@ -32,7 +32,7 @@ test:
     close($c);
     die qq{no total line in coverage.out\n} unless defined $total;
     printf qq{Total coverage: %s%%\n}, $total;
-    exit($total < 80 ? 1 : 0);
+    exit($total < 82 ? 1 : 0);
 
 # The same suite under the race detector. The expensive one.
 race:
@@ -100,6 +100,15 @@ toml-test: build
 coverage-html: test
     go tool cover -html=coverage.out -o coverage.html
 
+# Cross-compile smoke: the library and the command build for the foreign architectures; not a gate, it is a nightly convenience and runs std-lib only.
+cross:
+    GOARCH=arm64 go build ./...
+    GOARCH=loong64 go build ./...
+    GOARCH=riscv64 go build ./...
+    GOARCH=arm64 CGO_ENABLED=0 go build -o /dev/null {{package}}
+    GOARCH=loong64 CGO_ENABLED=0 go build -o /dev/null {{package}}
+    GOARCH=riscv64 CGO_ENABLED=0 go build -o /dev/null {{package}}
+
 # Runs the example program under examples/basic; not standard because `run` runs the adapter, and an example is documentation, not the product.
 example:
     go run ./examples/basic