feat: add --seed flag for deterministic manifest UUID

Adds a --seed CLI flag to 'generate' that derives a deterministic UUID
from the seed value by hashing it 1,000,000,000 times with SHA-256.
This makes manifest generation fully reproducible when the same seed
and input files are provided.

- Builder.SetSeed(seed) method for programmatic use
- deriveSeedUUID() extracted for testability
- MFER_SEED env var also supported
- Test with reduced iteration count for speed

internal/cli/mfer.go

@@ -154,6 +154,11 @@ func (mfa *CLIApp) run(args []string) {
 						Usage:   "GPG key ID to sign the manifest with",
 						EnvVars: []string{"MFER_SIGN_KEY"},
 					},
+					&cli.StringFlag{
+						Name:    "seed",
+						Usage:   "Seed value for deterministic manifest UUID (hashed 1B times with SHA-256)",
+						EnvVars: []string{"MFER_SEED"},
+					},
 				),
 			},
 			{