When a guest calls swactor.send then traps, the outbox entry survives
and leaks into the next successful handle() call. The outbox should be
cleared when handle traps, since the guest's operation was incomplete.
Also adds: invalid WASM bytes test, zero-length payload send test,
multiple sequential traps test (all passing).
Authored by Claude, lovingly guided by Zachery Aaron Shores-Chmielewski
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