remove time-hard hash iteration from seed UUID derivation

Per review: the iterated SHA-256 hashing (150M rounds) was unnecessary.
Now uses a single SHA-256 hash of the seed to derive the UUID.
Removed seedIterations constant, iteration loop, and all related
documentation/comments.

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"},
 					},
 				),