merge-check.sh runs and vets the integration level with the integration tag and times each level; the unit bound is not enforced yet (issue 895)

Makefile

@@ -110,14 +110,18 @@ proxy-image:
 # after formatting never ran through it. The script is the one list of steps; this target only gives it
 # a database (MESH_TEST_POSTGRES, exported above).
 #
+# **Two levels** (issue 895): the unit level, `go test ./...` untagged, timed and held to a bound; and the integration
+# level, `-tags integration`, the tests that start a bus, a store or another process. Each is said with its time.
+#
 # **Packages in parallel, under the race detector, each test on a bus of its own** (internal/testbus),
 # as the script runs them: a bus per test, of the release the mesh runs, makes the suite the same in any
 # order, and the timeout bounds a hang to a failure with a stack, never a stalled gate.
 check: postgres
 	@sh merge-check.sh ; status=$$? ; docker rm -f $(PG_CONTAINER) >/dev/null 2>&1 || true ; exit $$status
 
-# Without a database the store's tests skip rather than fail, so this is the honest subset and not
-# the gate.
+# The unit level alone (untagged: no bus, no store, no other process), which a developer runs continuously. The tests
+# that need a bus or the store carry `//go:build integration` and run in `check`, not here; without a database the
+# store's tests would skip rather than fail anyway.
 test:
 	go test -timeout 15m ./...