test: prove captured action stdin isolation

docs/spec.md

@@ -657,14 +657,23 @@ remaining separate actions. Reachable dependency initialization consequently
 observes the tested product's directory; a dependency tested as its own product
 observes its own directory.
 
+The same coordinator-executed product reads standard input from the null
+device. Its first read observes EOF regardless of the terminal, pipe, or file
+connected to the invoking command. Each parallel product owns a separate null
+descriptor, and production or test-only dependency initialization, filtering,
+listing, no-match execution, failure, and timeout all retain that boundary.
+Standard input is runtime process metadata and contributes no package, action,
+artifact, or persistence identity.
+
 The coordinator does not change its own cwd or environment. Parallel products
 receive independent child environments and each uses its own source directory.
 Relative ordinary files, `testdata`, and writes resolve there for every
 executing filter or list path. `ww build`, directory `ww test -c` (including
 `-c -o`), and a no-selected-test directory execute no test child and receive no
 execution-directory effect. A published test binary invoked directly, and the
-raw single-file compatibility route, inherit the user's invocation cwd and
-environment; no package directory is embedded or forced by the binary.
+raw single-file compatibility route, inherit the user's invocation cwd,
+environment, and standard input; no package directory or input policy is
+embedded or forced by the binary.
 
 ---