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.

mfer/builder.go

@@ -92,25 +92,12 @@ type Builder struct {
 	fixedUUID      []byte // if set, use this UUID instead of generating one
 }
 
-// seedIterations is the number of SHA-256 rounds used to derive a UUID from a seed.
-// Tuned to take approximately 5-10 seconds on modern hardware.
-const seedIterations = 150_000_000
-
 // SetSeed derives a deterministic UUID from the given seed string.
-// The seed is hashed 150,000,000 times with SHA-256 to produce
-// 16 bytes used as a fixed UUID for the manifest (~5-10s on modern hardware).
+// The seed is hashed once with SHA-256 and the first 16 bytes are used
+// as a fixed UUID for the manifest.
 func (b *Builder) SetSeed(seed string) {
-	b.fixedUUID = deriveSeedUUID(seed, seedIterations)
-}
-
-// deriveSeedUUID hashes the seed string n times with SHA-256
-// and returns the first 16 bytes as a UUID.
-func deriveSeedUUID(seed string, iterations int) []byte {
 	hash := sha256.Sum256([]byte(seed))
-	for i := 1; i < iterations; i++ {
-		hash = sha256.Sum256(hash[:])
-	}
-	return hash[:16]
+	b.fixedUUID = hash[:16]
 }
 
 // NewBuilder creates a new Builder.