reduce seed iterations to 150M (~5-10s on modern hardware)

1B iterations was too slow (30s+). Benchmarked on Apple Silicon:
- 150M iterations ≈ 6.3s
- Falls within the 5-10s target range

mfer/builder.go

@@ -93,11 +93,12 @@ type Builder struct {
 }
 
 // seedIterations is the number of SHA-256 rounds used to derive a UUID from a seed.
-const seedIterations = 1_000_000_000
+// 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 1,000,000,000 times with SHA-256 to produce
-// 16 bytes used as a fixed UUID for the manifest.
+// 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).
 func (b *Builder) SetSeed(seed string) {
 	b.fixedUUID = deriveSeedUUID(seed, seedIterations)
 }