Make the tests fast under the race detector (closes #120)

Deriving keys from passphrases with scrypt, slow on purpose, took most
of the test time under -race. secret.ScryptWorkFactor, when not zero,
replaces age's work factor when a passphrase encrypts; the tests of
internal/secret, internal/vault and internal/cli set it to 1 in
TestMain, and the program never sets it. TestGetCommandOutputsToStdout
checks that the built binary's passphrase unlocker names age's 18.

TestRemovalAsksWithoutHoldingLock and TestFailedCommandReleasesLock
time the in-memory lock all tests share, so they no longer run in
parallel. TestConcurrentAddsKeepEveryVersion and
TestGetCommandOutputsToStdout time nothing and now do.

The script/cibuild comment no longer says tests are skipped without
its memlock ulimit.

Model: opus-5-5

TODO.md

@@ -24,14 +24,16 @@ https://git.eeqj.de/sneak/secret/milestone/12
   `secret.ScryptWorkFactor`, when not zero, replaces age's scrypt work factor
   when a passphrase encrypts; the tests of `internal/secret`, `internal/vault`
   and `internal/cli` set it to 1 before any test runs, and the program never
-  sets it. `TestRemovalAsksWithoutHoldingLock` and
-  `TestFailedCommandReleasesLock` no longer run in parallel with other tests:
-  each waits at most 10 seconds for the in-memory lock that every test in the
-  package shares, and other tests' commands held it longer.
-  `TestConcurrentAddsKeepEveryVersion`, which times nothing, and
-  `TestGetCommandOutputsToStdout`, which no longer sets an environment variable
-  its commands do not read, now run in parallel. The `script/cibuild` comment no
-  longer says that tests are skipped without its memlock ulimit.
+  sets it; `TestGetCommandOutputsToStdout` checks that the passphrase unlocker
+  the built binary's `secret init` writes names age's work factor, 18.
+  `TestRemovalAsksWithoutHoldingLock` and `TestFailedCommandReleasesLock` no
+  longer run in parallel with other tests: each waits at most 10 seconds for the
+  in-memory lock that every test in the package shares, and other tests'
+  commands held it longer. `TestConcurrentAddsKeepEveryVersion`, which times
+  nothing, and `TestGetCommandOutputsToStdout`, which no longer sets an
+  environment variable its commands do not read, now run in parallel. The
+  `script/cibuild` comment no longer says that tests are skipped without its
+  memlock ulimit.
 - 2026-10-05: No test stores a secret larger than 1 MiB
   (https://git.eeqj.de/sneak/secret/issues/52). The size tests for `secret add`,
   `secret import` and the stdin buffer no longer try 2 MB, 10 MB, 99 MB, 100 MB

internal/cli/stdout_stderr_test.go

@@ -41,6 +41,18 @@ func TestGetCommandOutputsToStdout(t *testing.T) {
 	output, err := cmd.CombinedOutput()
 	require.NoError(t, err, "init should succeed: %s", string(output))
 
+	// The binary, unlike these tests, encrypts the passphrase unlocker's key
+	// at age's scrypt work factor, 18. age writes the work factor last on the
+	// second line of priv.age: "-> scrypt <salt> <work factor>".
+	vaultDir := filepath.Join(tempDir, "vaults.d", "default")
+	unlockerName := readFile(t, filepath.Join(vaultDir, "current-unlocker"))
+	unlockerDir := filepath.Join(vaultDir, "unlockers.d", string(unlockerName))
+	privAge := readFile(t, filepath.Join(unlockerDir, "priv.age"))
+	header := strings.SplitN(string(privAge), "\n", 3)
+	require.Len(t, header, 3, "priv.age should start with an age header")
+	assert.Regexp(t, `^-> scrypt \S+ 18$`, header[1],
+		"the passphrase unlocker should be encrypted at scrypt work factor 18")
+
 	// Add a secret
 	//nolint:gosec // G204: test executes the freshly built secret binary
 	cmd = exec.CommandContext(t.Context(), secretPath, "add", "test/secret")