test: the carried markup, by rebuilding its digest

Check that they are the right files: source.digest is the sha256 over
one line per markup file, "<SHA256> <basename>", sorted and newline
joined, so the same value can be computed from the stored rows alone and
held against what the .ssp recorded. A file stored under the right name
with the wrong contents passes a count and fails this.

Two details the shell gets wrong by default: the sort has to be
LC_ALL=C, because the writer sorts by byte and a locale does not, and
the lines are joined with no trailing newline, so printf and not echo.

(assisted by Claude-Code)

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