ww test: report no tests to run

docs/test-system-v2.md

@@ -222,12 +222,18 @@ run loop exactly. Measured on the 31-package `lib/...` walk:
 List mode uses that same product process and initialization boundary but starts
 no per-test child. The shared language harness emits only selected qualified
 test names, one per line in descriptor order. A valid filter selecting no tests
-therefore produces no harness list bytes, no `[no matches]` marker, and no
-accounting while the process returns success; the coordinator continues with
-the package's normal `ok` result. Ordinary non-list zero-selection execution
-retains its `[no matches]` marker and accounting. Concurrent products keep
-independent empty or nonempty captures, and persistent work never caches a test
-result.
+therefore produces no harness list bytes, warning, or accounting while the
+process returns success; the coordinator continues with the package's normal
+unsuffixed `ok` result. Ordinary non-list zero-selection execution instead
+writes the exact line `testing: warning: no tests to run` through harness
+stderr, retains its accounting and successful status, and gives the successful
+package `ok` result the suffix ` [no tests to run]`. Marker recognition is
+line-delimited, at capture byte zero or after a newline; an arbitrary substring
+in user output does not annotate the result. Concurrent products keep
+independent empty or nonempty captures, markers, and suffixes, and persistent
+work never caches a test result. A directly invoked retained binary exposes the
+warning on stderr and accounting on stdout because no coordinator combines its
+descriptors or emits a package result.
 
 Directory test binaries are always linked under the coordinator's temporary
 product root. `-c` independently requests a caller-visible executable copy and