fix(wasmguest): frame empty InvokeResponse as (ptr,0), not packed 0

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>

plugin/wasmguest/exports.go

@@ -77,10 +77,19 @@ func bnInvoke(hookID uint32, ptr uint32, length uint32) uint64 {
 	// bn_invoke returns). Release it whether or not the host calls bn_free.
 	delete(pinned, ptr)
 
-	lastResponse = resp
+	// A logically-empty InvokeResponse (Error nil + empty payload — e.g.
+	// LoadResponse/UnloadResponse) proto-marshals to ZERO bytes. That is a
+	// valid success envelope, not the "couldn't produce an envelope" sentinel,
+	// so frame it as (ptr, 0) with a real pointer. Returning packed 0 here
+	// would make the host mistake every empty-response hook (notably a
+	// successful LOAD) for an INTERNAL failure and discard the instance. A
+	// 1-byte backing keeps the pointer valid while the reported length stays 0
+	// (the host reads zero bytes → an empty InvokeResponse, which is correct).
 	if len(resp) == 0 {
-		return 0
+		lastResponse = []byte{0}
+		return uint64(bufPtr(lastResponse)) << 32
 	}
+	lastResponse = resp
 	return uint64(bufPtr(resp))<<32 | uint64(uint32(len(resp)))
 }