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.

mfer/builder.go

@@ -93,11 +93,17 @@ type Builder struct {
 }
 
 // SetSeed derives a deterministic UUID from the given seed string.
-// The seed is hashed once with SHA-256 and the first 16 bytes are used
+// The seed is hashed once with SHA-256 and the first 16 bytes are
-// as a fixed UUID for the manifest.
+// used as a fixed UUID for the manifest.
 func (b *Builder) SetSeed(seed string) {
+	b.fixedUUID = deriveSeedUUID(seed)
+}
+
+// deriveSeedUUID hashes the seed string with SHA-256
+// and returns the first 16 bytes as a UUID.
+func deriveSeedUUID(seed string) []byte {
 	hash := sha256.Sum256([]byte(seed))
-	b.fixedUUID = hash[:16]
+	return hash[:16]
 }
 
 // NewBuilder creates a new Builder.

mfer/builder_test.go

@@ -150,17 +150,15 @@ func TestBuilderDeterministicOutput(t *testing.T) {
 	assert.Equal(t, out1, out2, "two builds with same input should produce byte-identical output")
 }
 
-func TestSetSeedDeterministic(t *testing.T) {
+func TestDeriveSeedUUID(t *testing.T) {
-	b1 := NewBuilder()
-	b1.SetSeed("test-seed-value")
-	b2 := NewBuilder()
-	b2.SetSeed("test-seed-value")
-	assert.Equal(t, b1.fixedUUID, b2.fixedUUID, "same seed should produce same UUID")
-	assert.Len(t, b1.fixedUUID, 16, "UUID should be 16 bytes")
 
-	b3 := NewBuilder()
+	uuid1 := deriveSeedUUID("test-seed-value")
-	b3.SetSeed("different-seed")
+	uuid2 := deriveSeedUUID("test-seed-value")
-	assert.NotEqual(t, b1.fixedUUID, b3.fixedUUID, "different seeds should produce different UUIDs")
+	assert.Equal(t, uuid1, uuid2, "same seed should produce same UUID")
+	assert.Len(t, uuid1, 16, "UUID should be 16 bytes")
+
+	uuid3 := deriveSeedUUID("different-seed")
+	assert.NotEqual(t, uuid1, uuid3, "different seeds should produce different UUIDs")
 }
 
 func TestBuilderBuildEmpty(t *testing.T) {