A secret too large for the locked-memory limit fails with an error, not a panic (closes #145)

secret add, import and generate secret crashed with memguard's panic once a
value outgrew the locked-memory limit, and because the value was held twice,
half the usual 8 MiB limit was enough. secret now raises the limit to the hard
limit at startup. Each of the three commands refuses, before its first locked
allocation, a limit that leaves fewer than 8 pages for its keys and buffers.
The value is read a page at a time into a memguard.Stream, which keeps it
encrypted, and copied once into locked memory after its size is checked
against 100 MiB and the limit. It leaves the stream through Next, not Read:
Read re-encrypts a leftover piece, and memguard hangs if that runs out of
locked memory.

Model: opus-5-5

internal/cli/generate.go

@@ -72,6 +72,12 @@ func newGenerateSecretCmd() *cobra.Command {
 				return fmt.Errorf("failed to initialize CLI: %w", err)
 			}
 
+			// Before readSecretEnv, which makes the first locked allocation
+			err = checkLockedMemoryLimit(0)
+			if err != nil {
+				return err
+			}
+
 			destroySecrets := cli.readSecretEnv()
 			defer destroySecrets()
 
@@ -133,10 +139,16 @@ func (cli *Instance) GenerateSecret(
 		return errLengthTooSmall
 	}
 
-	var (
-		secretValue string
-		err         error
-	)
+	if length > maxSecretSize {
+		return errSecretTooLarge
+	}
+
+	err := checkLockedMemoryLimit(length)
+	if err != nil {
+		return err
+	}
+
+	var secretValue string
 
 	switch secretType {
 	case "base58":