feat: freeze a reported case and detect edited bodies

Content hash rather than mtime, because mtime is wrong in both directions.
Any sent destination freezes the whole case with no override: two desks
holding contradictory accounts of one incident is worse than a stale body.

The marker is tested for PRESENCE rather than truthiness. It is write-once
and monotonic, so an empty or malformed value is a half-written record of a
case a desk may already hold, and the safe direction is to refuse; only an
explicit null counts as absent. A non-mapping marker refuses too rather than
raising AttributeError out of a .get, since cli turns Frozen into a message
and a traceback into a bug report.

The hash pins utf-8 rather than leaving the encoding to the locale: it is
compared against a body read back off disk, possibly under a different LANG,
and for a sent destination it is the record of what was disclosed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LByBnw83xr9YP85nskzkyE

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