volt: beam: Q79 -- name one canonical directory manifest: manifest.py, with sha256_manifest.py retained for the sha256sum-compatible line format

Q79's done-means allows either retiring the overlapping surface or naming
which file is canonical, not both. Retiring sha256_manifest.py would break
test_sha256_manifest.py and the reply-01..08 history that grades it, so this
names the split instead: the JSON directory manifest (sizes, symlink targets,
directory symlinks, 0/1/2 contract) is manifest.py; the plain
`digest  path` text shape stays in sha256_manifest.py. One docstring, no
behaviour change.

CHECK: python3 -m unittest discover -s wizard-work/beam -p 'test_sha256*.py' -q

wizard-work/beam/sha256_manifest.py

@@ -1,5 +1,11 @@
 #!/usr/bin/env python3
-"""Generate and verify a SHA-256 manifest for a directory of files."""
+"""Generate and verify a SHA-256 manifest for a directory of files.
+
+Retained for the sha256sum-compatible `digest  path` line format only. The
+canonical directory manifest for this lane is `manifest.py`: it records sizes,
+symlink targets and directory symlinks, and carries the 0/1/2 exit contract.
+Use this file when a consumer needs the plain text shape; use that file otherwise.
+"""
 import argparse
 import hashlib
 import os