feat(balance): mutations can modify power value of mutant toxins, adding a vanilla use for Robust Genetics

[chaosvolt]

Purpose of change

Minor idea on my to-do list, basically making it so mutations (and by extension mods) can alter how mutations and thresholds interact with the whole "point value total that mutant toxins tries to mutate towards" system.

Describe the solution

C++ changes:

1. In character.cpp, added support for mutagen_target_modifier to mutation_value_map.
2. In mutation.cpp, Character::mutation_chances now factors in any bonuses/penalties from a mutation value called mutagen_target_modifier, that is used to increase or decrease the mutation point target used when mutating via toxins. Also added a debug message for testing.
3. In mutation.h, added definition for mutagen_target_modifier.
4. In mutation_data.cpp, added JSON support for mutagen_target_modifier to mutation_branch::load.

JSON changes:

1. Defined mutagen_target_modifier values for all threshold mutations. Idea here is the point bonuses pick the highest of the relevant category JSON's iv_hunger, iv_thirst, and iv_fatigue value. So basically, crossing a threshold gives you a head start on picking up better post-thresh mutations based on how physically disruptive that category is implied to be, since those tend to be the more spicy categories with a lot of high-risk-high-reward proporties. Most of these thresholds use 10 since hardcoded defaults are 10+10+5, as most categories don't define different values.
2. De-obsoleted Robust Genetics in mainline, now having a mutagen_target_modifier of +8. Re-allowed as a starting trait as it once was, with an updated description.
3. As I discovered that toxin mutations aren't affected by this (and for fun, for the matter), added a penalty of -50 to Genetic Downward Spiral.

Doc updates:

1. Documented new property in json_info.md

Describe alternatives you've considered

1. Not also re-mainlining Robust Genetics.
2. Making the bonus stronger.
3. Conversely, making the point cost lower than its old default of 3 since right now point-target mutations are only implemented for one source of mutation.
4. Adding it to Genetically Unstable and Genetic Chaos for bonus lulz.
5. Figuring out a way to hack the other mutation methods to actually factor mutation point cost in somehow. Could save for a followup tho.

Testing

1. Checked affected JSON for syntax and lint errors.
2. Compiled and load-tested.
3. Debugged myself a high duration of the mutating effect.
4. Observed that target value was default of 8, as expected.
5. Repeated test after giving myself Chimera threshold, target value was correctly reported as 28.
6. Re-tested with Genetic Downward Spiral instead, value was strongly negative as expected.
7. Checked affected C++ files for astyle.

Additional context

Checklist

[Coolthulhu]

8 points for 3 points is too good. Try 6.

10 points for thresholds is also a lot. It would be fine for bad trees, like plant, but not for medical or alpha.

[chaosvolt]

8 points for 3 points is too good. Try 6.

10 points for thresholds is also a lot. It would be fine for bad trees, like plant, but not for medical or alpha.

I wasn't sure what'd be good numbers to use, ye. That said, it's 10 for most categories, 5 for Alpha and Medical, 20 for Chimera.

Also note that in the case of Robust Genetics, 3 of that bonus it grants is effectively eaten up by its own point cost.

[chaosvolt]

I'll leave this one to wait until seeing whether @Coolthulhu would still prefer a particular nerf to the bonuses or not, given the aforementioned "part of the bonus is eaten up by the mutation's point bonus" thing.