ww test: report no tests to run

docs/spec.md

@@ -828,12 +828,18 @@ List mode starts the same directory-owned product and therefore performs its
 ordinary package initialization, but it does not start any selected test child.
 The harness emits exactly one newline-terminated qualified name for each test
 selected by the command's filters, in descriptor order. If the selected set is
-empty, the harness emits no list payload, no `[no matches]` sentinel, and no
-accounting, and returns success; the coordinator still emits the ordinary
-successful package result. This does not alter non-list execution: a valid
-filter selecting no tests continues to emit `[no matches]` and the
-discovered/selected/started/completed accounting. A retained binary invoked
-directly has no coordinator package result.
+empty, the harness emits no list payload, warning, or accounting and returns
+success; the coordinator still emits the ordinary successful package result
+without a no-tests suffix. Ordinary non-list execution is distinct: a valid
+filter selecting no tests writes exactly
+`testing: warning: no tests to run\n` through the harness's standard error,
+preserves the discovered/selected/started/completed accounting and successful
+exit, and causes a successful directory package result to end in
+` [no tests to run]`. The coordinator recognizes that exact warning only at
+capture byte zero or after a newline. Directory execution combines the warning
+and accounting in its product-local capture; a retained binary invoked directly
+keeps the warning on standard error, its accounting on standard output, and has
+no coordinator package result.
 
 After those ordered results, an ordinary explicit `ww test` request that records
 an attributable setup, build, or execution failure emits exactly one standalone