ww test: retain directory test binaries like Go

docs/spec.md

@@ -645,6 +645,27 @@ mixed directories are accepted from their selected test files. A directory
 with no selected test file validates ordinary production but creates no test
 support, generated main, link, binary, result, or process.
 
+Every test-bearing directory product links one request-private runnable.
+`-c` retains an executable copy and suppresses its execution; `-o` retains a
+copy at the named destination and still executes unless `-c` is present. With
+no explicit output, `-c` writes `<import-leaf>.test` in the invocation
+directory. An output ending in `/` or naming an existing directory receives
+that basename and may have missing parent directories created. One
+non-directory output may name only one package. Multiple packages whose
+visible import leaves would produce the same test-binary name reject before
+tools or output creation; exact `/dev/null` is the discard exception. Declared
+package names, test variants, source filenames, physical directories, output
+paths, and retained binary names remain presentation or loader metadata and do
+not become canonical package or action identity.
+
+The retained file is an executable, byte-identical copy of the private
+runnable. It joins package artifacts and statuses in the request-wide atomic
+publication transaction. Build, link, stage, or install failure preserves old
+destinations and removes temporary stages and invocation-created output
+prefixes. Test execution starts only after that transaction commits, so a
+runtime failure leaves an explicitly retained binary. A no-test product
+publishes no binary and does not create a directory solely for one.
+
 When `ww test` executes a directory-owned product, the child process working
 directory is that product's canonical absolute physical package source
 directory. Its per-run environment has exactly one effective uppercase `PWD`,