Harden maker notification layer: kind passthrough, webhook prefix/retry, config validation (fixes #221)

[wjllance]

Summary

Six small notification-layer hardening fixes for the maker bot, grouped per issue #221:

• PositionRiskKind passthrough: position_jump() now reads event.kind and emits distinct kinds (direction_flip / max_position_crossed / inventory_exit_crossed / position_jump), escalating DirectionFlip and MaxPositionCrossed to severity critical. Crossing max_position is no longer indistinguishable from an ordinary 20% jump.
• Webhook text prefix: the phone webhook now gets the same [severity/kind] symbol: prefix the stderr line carries, so alerts identify their source across multiple instances.
• Reject unknown config keys: MakerFileConfig gains #[serde(deny_unknown_fields)], so a typo like alert_los errors at load instead of silently leaving the guard off.
• Threshold range validation: alert_loss / alert_inventory_pct / alert_uptime are validated at startup (negatives and percentages >100 rejected) via a pure validate_alert_thresholds helper, mirroring the existing alert_position_change_pct check.
• Order-response reconnect notifications: the order-response stream now emits risk_notification on disconnect, reconnect success, and reconnect failure, matching the account-stream path (previously entirely silent on the webhook).
• Webhook retry + await-capable alert: post_webhook retries twice with linear backoff on 5xx/timeout; alert() gained an await-capable path so a firing alert on the final cycle before shutdown is not dropped with its spawned task.

Test plan

• cargo fmt -- --check (no diff)
• cargo clippy --workspace --all-targets -- -D warnings (clean)
• cargo test -p standx-cli and cargo test -p standx-maker (all pass)
• New unit tests: risk_kind_descriptor mapping/severity escalation, deny_unknown_fields typo rejection, and validate_alert_thresholds range rejection.

Fixes #221

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