test(derive): close first mutation shard

Create shard output parents, exercise exact handler placement/signature/codegen contracts, preserve generated-file and form-bound diagnostics, and fail closed on malformed form symbol manifests. Public shard 1/8 passes through both 16-way partitions.

req: derive_handler/001

req: client_local/001

req: form/004

req: diagnostics/003

req: test/022

req: test/023

hemx-xtask/src/main.rs

@@ -1347,6 +1347,15 @@ fn run_mutation_plan(package: Option<&str>, shard: Option<&str>) -> ExitCode {
                     .join(format!("shard-{}", shard.replace('/', "-of-")))
             },
         );
+        if let Some(parent) = output.parent() {
+            if let Err(error) = fs::create_dir_all(parent) {
+                eprintln!(
+                    "failed to create mutation output parent {}: {error}",
+                    parent.display()
+                );
+                return ExitCode::FAILURE;
+            }
+        }
         let jobs = budget.jobs.to_string();
         let mut command = Command::new(&mutest);
         command