ams-calibration: aggregate backtest track record into the miner calibration report + snapshot event

[JSONbored]

Problem

ORB's #8140 taught us the loop is only trustworthy once REGRESSED verdicts accumulate a track record. The miner's calibration surfaces (lib/calibration.ts buildCalibrationReport, the MINER_CALIBRATION_SNAPSHOT_EVENT written each cycle) know nothing about backtest runs, so an operator cannot see whether the advisory backtests are earning authority.

Requirements

⚠️ Required pattern. Reuse computeRegressedVerdictTrackRecord from @loopover/engine over the persisted AMS backtest-run events — same aggregation ORB uses, zero new math.

• Extend buildCalibrationReport with a track-record section aggregated from the miner_event_ledger backtest-run events (the threshold-backtest issue's persistence).
• Include the track record in the periodic calibration snapshot event so history is queryable from the ledger alone.
• CLI: the existing calibration report command prints the section; empty history renders an explicit 'no backtest runs recorded' line, never a blank.

Deliverables

• Track-record section in the report + snapshot with 100%-branch coverage on new pure logic
• Pack-test coverage for the ledger read path

Links & Resources

#8172 (epic), #8140 (ORB analog), packages/loopover-miner/lib/calibration.ts, lib/loop-cli.ts (MINER_CALIBRATION_SNAPSHOT_EVENT)

Boundaries

Read-and-aggregate only; no knob movement.

maintainer-only — epic decomposition of #8172.

[linear-code]

JSO-5683