stringify false? and ~(false?) to falsy and truthy respectively

[nnullcolumn]

soo this is my first time flagging a change, not confident i did that correctly tbh but tests seem to run okay

• changes false? to falsy and ~(false?) to truthy in stringification
• added fflag / tests, ran on NixOS 26.05 x86_64 w/ make test
• related add truthy and falsy built-in type aliases #2516
• related built-in type aliases for truthy & falsy rfcs#224
• i added some untracked files to .gitignore for convenience w/ my server's clangd setup... if those need to be removed from this pr thats fine
• this code was 100% human written

[nnullcolumn]

i have been informed i did not, infact, flag it correctly :p

edit: will fix soon, busy w some stuff for now

[nnullcolumn]

^ full disclosure, i had an AI agent do the commit updating tests to switch on the fflag everywhere. I went through the diff line by line and everything looks good. It also spotted a mistake, I didn't look closely enough at the cycle detection stuff here and placed the early return for falsy after the check for state.hasSeen, but didn't check if inserting to the seen set was a separate operation (it wasn't).