stackyard fleet --json and stackyard audit --json #2

./stack list and ./stack --check answer in JSON with --json (v0.31.0), so an agent or a monitor can read them without parsing text. The two fleet-wide commands cannot yet.

What to build:

• stackyard fleet --json: {"stackyard":{"version","commit"},"machines":[{"name","path","version","commit","behind"}]} — behind a number, or null when it cannot be counted.
• stackyard audit --json: the same shape ./stack --check --json uses — {"command","ok","problems","warnings","findings":[{"section","level","message"}]}.
• In both, stdout carries the JSON and nothing else; the exit code keeps its meaning.

Where to look:

• platform/lib/lib-env.sh, the JSON section: json_str, json_list, json_finding, json_findings_object, written without jq.
• platform/bin/stack.sh: how --json moves the human report to stderr (exec 3>&1 1>&2) and records findings in ok/warn/bad.
• bin/fleet.sh, bin/audit-isolation.sh.

Done when: selftest parses both outputs with a real JSON parser (see the "== --json" block) and compares the counts with the findings.