[core] add onMobSkillReadyTime to choose ready time for mobskill

[WinterSolstice8]

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• I have read and understood the Contributing Guide and the Code of Conduct.
• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

Addresses part of #9372 (Meltdown is a separate subject)

return nil (or dont return anything, which counts as nil) to "not decide" and use the default time
return a number to change the ready time

Steps to test these changes

Add

entity.onMobSkillReadyTime = function(target, mob, skill)
    return 0
end

to a mob of choice, such as a huge hornet...
(I gave it 3000 regain)

2026-02-05.09-06-16.webm