Add --stop-loss and equity/margin threshold alerts (fixes #219)

[wjllance]

Summary

The maker bot had no automatic financial brake. alert_loss only sent a notification while the bot kept quoting, and account equity / available margin were fetched every cycle but never compared to any threshold — liquidation risk was watched only by a human.

This PR adds three opt-in financial guards:

• --stop-loss <X> — when session mark-to-market PnL breaches -X (quote units), the bot routes through the existing fail-safe shutdown path (freeze, cancel the maker book via cancel_maker_orders_with_retry, await the critical webhook, exit) using a new MakerExit::StopLoss variant. No new shutdown mechanism is introduced. The check runs in the runtime cycle loop over the just-updated stats.
• --alert-equity-below <X> — edge-triggered alert when account equity drops below X.
• --alert-margin-below <X> — edge-triggered alert when available cross margin drops below X.

The account snapshot already fetched each cycle is threaded out of the cycle (CycleResult.balance) and compared in AlertMonitor::evaluate_account with a 10% recovery hysteresis to avoid flapping. The two account alerts are live-only (no account snapshot in paper mode).

All three flags flow CLI → file → default via the existing choose() pattern (cli.rs, mod.rs MakerRunArgs, config.rs MakerFileConfig). The STANDX_ENABLE_LIVE_MAKER live lock is untouched.

Test plan

• cargo fmt -- --check (clean)
• cargo clippy --workspace --all-targets -- -D warnings (clean)
• cargo test -p standx-cli -p standx-maker (all pass)
• New unit tests:
  • AlertMonitor equity-floor edge fire/clear with hysteresis, and margin-floor firing independently (standx-maker)
  • MakerExit::StopLoss lifecycle/terminal messaging (standx-cli)
  • config parsing of stop_loss, alert_equity_below, alert_margin_below
• Stop-loss is verifiable in paper mode: paper fills feed MakerStats, so an adverse mark drives stats.pnl below -stop_loss and the fail-safe shutdown fires.

Fixes #219

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