feat: JSONize spellcasting encumbrance threshold

[Hourousha]

base_leg_encumbrance value when calculating how much encumbrance is ignored for casting a spell instead of the previous hard-coded 20. The JSON value is optional, and it defaults to 20.

Purpose of change (The Why)

Casting a spell using encumbrance ignored the first 20 points of arm and leg encumbrance in magic.cpp. 20 is a hard-coded value.

Describe the solution (The How)

The values are now optional JSON fields (named base_arm_encumbrance_value and base_leg_encumbrance_value) per spell with a default value of 20. No documentation was added for the field.

Describe alternatives you've considered

Testing

I verified the fields are accepted when input as values for spells in JSON.

Checklist

Mandatory

• I wrote the PR title in conventional commit format.
• I ran the code formatter.
• I've committed my changes to new branch that isn't main so it won't cause conflict when updating main branch later.
• I understand that, unless specified otherwise, my contributions will fall under the AGPL v3.0 and/or CC-BY-SA 4.0 licenses

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[RobbieNeko]

Code looks good to me, now to compile and test

[chaosvolt]

1. Compiled and load-tested.
2. Loaded in a Magical Nights save as a novice stormshaper.
3. Observed my failure rate with Windrunning does not go up any when equipping a riot shield (10 enc on only a single arm) nor even a banded shield (20 enc on one arm).
4. Observed that failure rate starts to go up if I put on arm warmers over said banded shield, increasing total enc from 20 to 22 (1 per arm).

I was once again almost bamboozled by the function being goofy and counting the encumbrance of each limb independently.

[RobbieNeko]

Looks good! Compiled, and made sure that it works on my machine properly. Even tested setting the threshold manually above the default 20 and it worked.

Only suggestion is just related to the discussion in the Discord about making the fields more sensibly named

[RobbieNeko]

Suggested change

	optional( jo, was_loaded, "base_arm_encumbrance_value", base_arm_encumbrance_value, 20 );
	optional( jo, was_loaded, "arm_encum_thresh", arm_encumbrance_threshold, 20 );

Perhaps this would be a better set of names? And similar can go for the leg values too

[chaosvolt]

If anything we could even unify it to use just one value for all properties but, which would OP prefer I guess?

[Hourousha]

Pursuant to feedback, I changed the variable and JSON names to arm_encumbrance_threshold and leg_encumbrance_threshold. I searched the code and encum is less frequent in the code base than encumbrance. Honestly, my preference is just using one (either encum or encumbrance) everywhere because I personally can't remember which to use where when there's more than one (sometimes, I still struggle when there's just one). I completely defer to you as I can't even grok a lot of the C++ code base.

[Hourousha]

If anything we could even unify it to use just one value for all properties but, which would OP prefer I guess?

Personally, I'm confused. Arm encumbrance is OPT-IN (it won't count unless the spell_flags JSON SOMATIC is set, but leg encumbrance is OPT-OUT (it's will count unless the spell_flags JSON NO_LEGS is set), so I'm completely confused about unifying the values.

[chaosvolt]

Whoops, didn't see the reply to this. Seems fine then.