Allow auto-equipping items when the inventory item frees space for the equipped item

[ephphatha]

New behaviour: https://github.com/user-attachments/assets/3b7ab495-59f9-4315-a986-18a8b876c40d

Main thing of note is the item that used to be equipped will fill the gap left by the item being equipped if it fits.

Commit 2324108386c74758f20745a932768a2917a12bb5 is some prep that made it easier to reason about the steps required, I tried to keep the same behaviour/sound feedback. What I was attempting to show at about 10 seconds into this clip was trying to equip the short sword when there's no room to remove the two-handed sword would play the "I have no room" speech but the screen recording didn't capture sound :(

This builds on #7494, I've split them to hopefully make things easier to review.