Architecture decision records

An ADR records a decision that constrains future work: the context that forced it, the choice made, and the consequences accepted. An accepted record is not edited; a reversal is a new record that supersedes it.

Add docs/adr/NNNN-slug.md with a # NNNN — Title heading and a - **Status:** line, then run cargo run -p xtask -- docs: the index below and the sidebar follow from the file.

#	Title	Status
0001	IFC5 / IFCX is its own standard family	Accepted
0002	Split crates by dependency weight, not by concept	Accepted