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Two fixes to how probbit live verify and probbit explain answer damaged or unusual strands, a test that damages a
strand one place at a time, and the documented exit codes of explain. No change to strand formats, digests or the
output for a healthy strand.

Stacked on #94 (fix/plugin-checkpoint-tail).

• No panic on a non-ASCII persona digest (probbit-persona/src/persona.rs). When a state's persona.digest does
  not match the persona, the error message showed its first 19 bytes. A digest with a multi-byte character at that
  offset (for example ten é) made live verify and explain panic (exit 101). The message now cuts by
  characters; both commands exit 1 and name the mismatch (live verify and explain --json print it as a JSON
  document). This was in 0.14.0 too.
• A bounded no-panic sweep (probbit-cli/tests/damaged.rs). The test writes a small strand with the engine (the
  sensed example's first 3 events, a checkpoint after the 2nd: 5 lines) and makes every variant with one damage: each
  string value of each line set to ten é, to "" and to "x", and each line cut in the middle. That is 467
  variants; each goes through live verify, explain --event 1 and explain --event 3 (1,401 runs, about 7 seconds
  on a debug build). Every run must exit 0, 1, 2 or 5 with one JSON document on stdout; a run that panics, dies of a
  signal or takes longer than 20 seconds fails the test. The variant count is capped at 600 with an even stride. With
  the digest fix reverted the sweep reports exactly three failures, all the panic above; with it, none.
• explain lists wrapped Boolean sense readings (probbit-cli/src/explain.rs). A reading may be bare
  ("tool_fail": true) or wrapped ({"v": true, "src": "env:runner"}, docs/persona.md section 2.11; v and src are
  the only keys the engine accepts). explain looked for a key value, so a wrapped Boolean was missing from
  flags. It reads v now; a test checks both forms and fails without the fix.
• explain's exit codes as they behave (CHANGELOG, docs/CHEATSHEET.md). 1: the selected event, or a line before
  it, does not recompute, as live verify reports it (a changed digest, a broken chain, inputs of an earlier event
  that the engine rejects, a damaged copy of the sealing line in a segment file). 2: a bad flag or out-of-range
  --event, an unreadable file, a line that is not JSON or an event line without its fields, or inputs of the
  selected event that the engine rejects. The behaviour is unchanged; the text said "1 differs, 2 bad input". The
  command's help lists no exit codes, so the help and the man page are unchanged.

Validation on the host (macOS arm64), run on this branch's Rust code and tests before its last rebase (the rebase
changed only the plugin below it, in #94):

• Rust cargo test --offline -p probbit-cli: 308 passed, 0 failed, 3 ignored (305 before; 3 new tests).
• Rust cargo test --offline -p probbit-persona: 81 passed, 0 failed, 1 ignored (at the commit with the digest fix;
  later commits touch no persona code).
• Plugin npm test against this branch's release build: 165 tests (from fix(openclaw): read a strand's tail past checkpoint lines; pause a sealed lane #94); the only failures seen were the known 500 ms
  spawn-budget timing tests.
• scripts/generate_llms_full.py --check passes.