A successful hook whose response message has no set fields (LoadResponse,
UnloadResponse) proto-marshals to zero bytes. bn_invoke's `len==0 → return 0`
shortcut collided with the "callee could not produce an envelope" sentinel, so
every successful empty-response hook — notably HOOK_LOAD — looked like an
INTERNAL failure and got the instance discarded. Frame the empty case as
(ptr, 0) with a real 1-byte-backed pointer instead; the host reads zero bytes
into a valid empty InvokeResponse.
Surfaced by the WO-WZ-003 end-to-end capability test (first exercise of LOAD).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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