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_test.go

@@ -150,15 +150,17 @@ func TestBuilderDeterministicOutput(t *testing.T) {
 	assert.Equal(t, out1, out2, "two builds with same input should produce byte-identical output")
 }
 
-func TestDeriveSeedUUID(t *testing.T) {
-	// Use a small iteration count for testing (production uses 1B)
-	uuid1 := deriveSeedUUID("test-seed-value", 1000)
-	uuid2 := deriveSeedUUID("test-seed-value", 1000)
-	assert.Equal(t, uuid1, uuid2, "same seed should produce same UUID")
-	assert.Len(t, uuid1, 16, "UUID should be 16 bytes")
+func TestSetSeedDeterministic(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")
 
-	uuid3 := deriveSeedUUID("different-seed", 1000)
-	assert.NotEqual(t, uuid1, uuid3, "different seeds should produce different UUIDs")
+	b3 := NewBuilder()
+	b3.SetSeed("different-seed")
+	assert.NotEqual(t, b1.fixedUUID, b3.fixedUUID, "different seeds should produce different UUIDs")
 }
 
 func TestBuilderBuildEmpty(t *testing.T) {