remove time-hard hash iteration from seed UUID derivation

Replace 150M SHA-256 iteration key-stretching with a single hash.
Remove all references to iteration counts, timing (~5-10s), and
key-stretching from code and documentation.

The seed flag is retained for deterministic UUID generation, but
now derives the UUID with a single SHA-256 hash instead of the
unnecessary iterative approach.

internal/cli/mfer.go

@@ -156,7 +156,7 @@ func (mfa *CLIApp) run(args []string) {
 					},
 					&cli.StringFlag{
 						Name:    "seed",
-						Usage:   "Seed value for deterministic manifest UUID (hashed 150M times with SHA-256, ~5-10s)",
+						Usage:   "Seed value for deterministic manifest UUID",
 						EnvVars: []string{"MFER_SEED"},
 					},
 				),