v0.3.0: reproducible peak-RSS evidence

Adds benchmark_memory.py, a dependency-free Linux benchmark that compares the working-tree CLI with v0.1.1 using sparse 1, 16, and 64 MiB inputs. It verifies output SHA-256 values and exercises the 64 MiB + 1 negative case.\n\nObserved on Linux/Python 3.12.3 in the clean release commit: at 64 MiB, v0.1.1 peaked at 84,352 KiB RSS and v0.3.0 at 20,688 KiB. For 64 MiB + 1, both exited 2, preserved the old destination, and left no temporary files; peaks were 84,352 KiB and 20,560 KiB. Unit suite: 8/8 passed in 0.466 s.\n\nThe first benchmark draft assumed GNU time, which was absent in the dogfood environment. The released benchmark uses Python resource.getrusage in an isolated worker instead. Measurements are same-host observations, not fixed guarantees or throughput results. Power-loss, NFS, full-disk, ACL/xattr, locking, idempotency, and consensus guarantees remain explicitly out of scope.