feat(amico-run): attribute CPU cost to principal and Lab from the solve unit cgroup, not process.cpuUsage() #2133

Why

Central-hub cost ledger (ADR 0041 section 5). It replaces the approach in #2121, which measured process.cpuUsage(): that counts only the engine process, not the Julia child, so every solve would be costed at about zero. Since A1, each solve runs in its own transient unit amico-run-<id>.service, whose cgroup accounts the solve's real CPU including all child threads.

What

1. Add principal, lab and cpu_seconds to ReceiptRecord (and ledger-record.schema.json), plus a costTotals rollup by principal and Lab. Unattributed runs roll up under (unattributed).
2. Measure CPU from the unit's cgroup. On Linux read cpu.stat usage_usec of the run's cgroup (find it via the A1 run.lock unit name, or systemctl --user show -p ControlGroup), read before the unit is torn down and again at settle; the delta is cpu_seconds. Where no cgroup is available (macOS, a run not in a unit), fall back to child rusage if obtainable, else record cpu_seconds absent and cost_basis: unmeasured. NEVER fall back to process.cpuUsage().
3. Cost = cpu_seconds x a rate read from a human-edited file (~/.amico/amicode/rates.json), default 0, so an unconfigured install records seconds and no invented dollars. Label every dollar figure basis: list_price_estimate.
4. Never fail a run because accounting failed.

Acceptance Criteria

• a test runs a real child process that burns CPU in a transient cgroup or unit (or a faithful temp cgroup fixture) and asserts cpu_seconds reflects the CHILD's CPU.
• a test proves process.cpuUsage() is never used: a run whose engine process is idle but whose child burns 1+ s must record at least 1 s.
• no rate configured => seconds recorded and no cost_usd; rate configured => cost_usd = seconds x rate with basis list_price_estimate.
• unattributed runs roll up under (unattributed); principal and Lab rollups sum to the total.
• an accounting failure never fails a run or changes its exit code.
• the schema validates the new fields.