Adds a --seed CLI flag to 'generate' that derives a deterministic UUID
from the seed value by hashing it 1,000,000,000 times with SHA-256.
This makes manifest generation fully reproducible when the same seed
and input files are provided.
- Builder.SetSeed(seed) method for programmatic use
- deriveSeedUUID() extracted for testability
- MFER_SEED env var also supported
- Test with reduced iteration count for speed
Sort file entries by path (lexicographic, byte-order) before
serialization to ensure deterministic output. Add fixedUUID support
for testing reproducibility, and a test asserting byte-identical
output from two runs with the same input.
Closes#23
