Dispatch returns a result envelope

dpm_execute fills a dpm_result on every call: status and error_code
from the module's return value, and a payload the module hands back
through dpm_set_result. The payload's layout belongs to the module and
is documented per command; its release function is carried in the
envelope and invoked by dpm_result_release. The context keeps a stack
of the envelopes in progress, so a module calling a peer receives the
peer's payload in its own envelope and its caller sees only what the
module sets itself.

The good fixture returns a payload and counts its releases, and the
modules test covers the envelope fields, the payload round trip, single
release, discard on a NULL envelope, and a payload set outside any
dispatch. The prose documents describe the envelope and the payload
header a module ships.

CMakeLists.txt

@@ -27,6 +27,7 @@ add_library(dpm-core SHARED
         src/core/logging.cpp
         src/core/metadata.cpp
         src/core/modules.cpp
+        src/core/results.cpp
         src/core/sanitizers.cpp
         src/core/version.cpp
 )