fix(stash): return top-level response fields

[SajoLuo]

What changed

• Return status, headers, and body at the top level for Stash response scripts.
• Preserve the existing nested response output used by request-script-style clients.
• Add a Node VM regression test that executes the distributed script as Stash 3.2.5.

Why

Stash only accepts response as a request-script field used to synthesize a response. A response script must pass status, headers, and body directly to $done. The existing finalizer wrapped the patched WLOC response in { response: ... }, so the script logged successful protobuf patches but Stash did not apply the modified response.

Impact

Stash users receive the patched WLOC binary response instead of the original response being left in place. Other clients keep their existing output shape.

Related to #5, #44, and #45.

Validation

• cd worker && npm test
• VM simulation verifies the Stash $done payload has no response wrapper and retains status, headers, and binary body.
• Confirmed against Stash 3.2.5 on-device.