Equip carried shields and acquire backpacks through discovered shops

[adrunkhuman]

• Let Knights equip carried shields without weakening melee-weapon restrictions.
• Discover standard backpack sellers through the existing shop catalog; no hardcoded NPCs. Buy a backpack for an empty back slot, or upgrade a basic bag through a depot → shop → depot trip that keeps the old bag and its contents intact.
• Track pending bag retrieval through normal player storage and item persistence. Recover interrupted upgrades, including death-created replacement bags, without dropping belongings on the ground.
• Add deterministic equipment-policy and purchase/recovery state-machine coverage.

Bag upgrades require banked funds for the purchase plus the configured reserve. Other container upgrades are out of scope. Supplies inside the staged bag are unavailable during the shop trip.

The full depot round-trip and death/reload recovery still need loaded-world verification; the initial live evaluation rejected the seller as unreachable before staging the bag.