oops-libs documentation

The host-side Rust crates every OOPS tool shares: build stamp, logging, paths and embedded documentation.

• The repository README - the crates, what each costs, and how to build.
• USER_GUIDE.md - using each crate in a tool.
• DECISIONS.md - the decisions in force, generated from decisions/ by tools/split-decisions.sh --index oops-libs in the collection. Never edit it by hand.

Shared rules are in the collection's CONVENTIONS and STYLE. Vocabulary is the collection's glossary; host, the one word this repository turns on, is defined in CONVENTIONS section 2.