Displaying non damaging numerical effects in spell description

[Slacking]

I’m mainly thinking about Passwall’s AC bonus, but I can imagine there are other spells that would gain from this as well.

My thought was to add spell_effect_string in spl-cast.cc that can then be used in describe.cc:_player_spell_stats and similar places.

[Slacking]

Could look something like this. Would look neater if passwall was updated to fractional AC but that is out of scope for this