docs: add flat remaining port features list to README

[Copilot]

README had no quick-scan summary of outstanding work — all detail was buried in PORT_STATUS.md.

Adds a "Remaining Port Features" section near the top of README.md with one line per feature, grouped into three buckets:

• In Progress / Broken — stubs and partial implementations (HumanoidNPC brain, targeted mob AI, wall decoration, item specializations, unregistered menus, empty BER stubs, SBL shim, VillagerBrainConfig CORE placeholder)
• Legacy / Deferred — functional but non-compliant systems (world gen, raw NBT, armor rendering, A* pathfinding, diplomacy logic)
• Not Started — dynasty, raids, quests, pujas, age progression, import table loop

Cross-references PORT_STATUS.md for full per-item detail.