Wheeler phase 2: automated early-assignment detection from Flex

[heyitsStylez]

Part of #106 (Wheeler phase 2). Rides in with IBKR Flex import (depends on the api/ibkr-flex.js ticket). Per #83, manual ASSIGNED-with-date is the v1 story; this automates detection.

Goal

Detect early assignment of short options from the Flex activity feed and set the correct outcome/date automatically, instead of the user marking ASSIGNED by hand.

Scope

• From imported Flex activity, identify assignment events (option position closed via assignment / resulting stock leg) and map them to the local OPEN option lot.
• Set outcome to ASSIGNED with the actual assignment date (may be before expiry — early assignment).
• Reconcile with the lot engine so the resulting equity lot / cost basis is correct.
• Tests covering an early-assignment Flex fixture end-to-end.

Done when

• A Flex import containing an early assignment flips the matching OPEN lot to ASSIGNED with the real date, no manual entry.
• npm test green.