Fix the lumbermill trader

[MNG-cataclysm]

Summary

Bugfixes "Fix a transactional error with the lumbermill trader"

Purpose of change

Like the chemist trader (#63064), the lumbermill trader had a bug in which purchase options would become unavailable if the player could not pay for them, as though they had bought the merchandise.

Describe the solution

Like the chemist, I moved the cooldown timer application to a true-false EOC, defined in the u_buy_item effect.

Describe alternatives you've considered

This is the alternative, mainly for #62804.

Testing

Went to a lumbermill, tried to buy some logs, and exited the trade menu when I couldn't afford it. The option remained available for purchase, and I successfully bought them later with some valuables. I repeated this with the other options that have cooldown timers.

Additional context

None.

[cake-pie]

An issue in data/json/npcs/lumbermill_employees/TALK_lumbermill_fabricate.json that probably originates from here seems to be causing Text Changes Analyzer checks to fail across multiple PRs that are completely unrelated to that file. (example)

> Extracting strings from JSON
Exception when parsing JSON data type "effect_on_condition"
Error in JSON file: 'data/json/npcs/lumbermill_employees/TALK_lumbermill_fabricate.json'
'effect'
Error in extracting strings from JSON. Aborting.