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.

tests/CMakeLists.txt

@@ -14,6 +14,10 @@ foreach(fixture good missing_symbols bad_version)
     )
 endforeach()
 
+# The known-good fixture returns a payload through the library, so it
+# links libdpm-core.so the way a module does.
+target_link_libraries(fixture_good PRIVATE dpm-core)
+
 # ---------------------------------------------------------------------
 # API test binaries — one per area of the library, each its own ctest
 # case, so a failure names the area it happened in