Fix duplicate inventory removal when equipping items

[keepithandy]

Goal

Make equipment changes safe when the inventory contains multiple copies of the same item id.

Current behavior

equipItem(itemId) removes equipped items with a full inventory filter, which can remove every copy of that item id instead of only the selected copy.

Proposed work

• Change equip behavior so equipping one item removes only one matching inventory entry.
• Preserve the existing behavior where the previously equipped item returns to inventory.
• Keep item ids stable and keep the inventory format as an array of item ids.
• Add or update smoke coverage for duplicate inventory entries.

Acceptance checks

• If inventory contains two copies of the same item, equipping one leaves the other copy in inventory.
• Replacing equipped gear still returns the previous item to inventory.
• Selling behavior remains unchanged.
• node smoke_depth_engine_core.mjs passes.
• node smoke_rat_cellar_content.mjs passes.

Guardrails

This is a focused inventory correctness fix. Do not rewrite the inventory model or introduce item instances in this issue.