fix: ally-only spells no longer autotarget enemies, fix crash when casting SWAP_POS on a tile with no creature to swap with

[chaosvolt]

Checklist

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Purpose of change

This fixes a couple long-running annoyances in spellcasting code I figured out solutions for. Also technically opens up unrestricted controlled teleportation as something spells can do without even needing a new spell function.

Fixes #4216

Describe the solution

C++ changes:

1. In magic_spell_effect.cpp, changed swap_pos so that it only tries to move any creatures found in the epicenter if there actually is one. If the tile targeted is empty, now instead of the game just imploding like before, it simply warps you over there without any complaints, meaning you can freely use SWAP_POS spells on any tile you want if you write the spell's JSON to permit targeting the ground to just use it for teleportation if you so choose.
2. In ranged.cpp, set target_ui::choose_initial_target so that spells don't try to autotarget any enemy in range unless they can ACTUALLY be used on hostiles, so you won't be prompted to try and waste your time casting a spell only usable on allies, and likewise making it less annoying trying to heal or buff yourself in the middle of a fight via whatever's kicking your ass trying to run off with your cursor.

Documentation changes:

1. Updated the explaination of what SWAP_POS does in magic.md.

Describe alternatives you've considered

Adding a spell in Magiclysm that uses SWAP_POS as a full-on blink spell. Robbie and Fox said they have plans already so will leave it to them.

Testing

1. Compiled and load-tested.
2. Started up a world with Magiclysm in it as a novice stormshaper.
3. Tested against both the starter NPC and a zombie, Lightning Bolt will highlight these targets automatically if in range but Windrunning no longer tries to target them.
4. Tested giving myself Holographic Transposition after temporarily removing the monster restriction and letting me cast it at the ground.
5. Cast this modified spell and I warp over to the selected tile without any crashes this time, even if the tile is empty.
6. Zapped starter NPC with it to confirm it also still works fine to just swap with the target.
7. Also tested temporarily giving Holographic Transposition damage while it was rigged to let me tag an NPC with it, doing so still lets me swap places with them, and simply teleports me on top of their corpse if the damage was lethal.
8. Checked affected files for syntax and lint errors.

Casual bullying of an NPC with a version of Holographic Transposition temporarily given damage, and it working without issue when the victim dies from it:

Additional context

Checked in DDA, as of build 2024-08-30-1256 targeting an invalid tile with a SWAP_POS spell still crashes the game:

Even better, a basic attempt to test and confirm if you still auto-target non-allies with Windrunning revealed that the spell I selected on the spellcasting menu ends up not being the spell it tries to cast:

So I have to try and cast Shocking Lash instead, which ACTUALLY casts Windrunning, and only then can I confirm DDA auto-targets non-allies with ally-only spells.

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[RoyalFox2140]

Simple enough, I'm going to be testing it later.

[chaosvolt]

One thing though: maybe I should also make it so SWAP_POS also doesn't teleport the user if the destination is impassible?

[RoyalFox2140]

I don't know about that. I think that's bad for a few reasons, assuming you can target an unknown area:

1. You can use it to fish and see where walls are in an unknown area.
2. You can't see where you should be able to stand in this case, so you'll have a lot of trial and error finding a spot to teleport to.

[chaosvolt]

That's what I mean, I'm asking if I should make it also check whether you're going to get yourself stuck in a wall and if so don't move the caster.

[RoyalFox2140]

That's what I mean, I'm asking if I should make it also check whether you're going to get yourself stuck in a wall and if so don't move the caster.

I meant that fixing it would make it more annoying to get a valid spot to teleport to without clairvoyance and that you could still use it to fish for where walls are. Getting stuck in a wall doesn't generally kill players as I do it with debug and it doesn't seem to cause damage, but maybe there's something in the code I don't know about. Worst case is you teleport back out.

[chaosvolt]

Ah, fair enough then. Letting the player teleport themselves into walls is easily fixed by just teleporting back out again, or walking out if a valid space is nearby.