fix(standalone): correct paths for submodule context + polish README

- build.rs: check affinescript/_build/ (standalone submodule) before
  ../../_build/ (monorepo dev path). Both paths tried; env var and PATH
  still take priority.
- justfile: add setup/bootstrap/update-affinescript recipes for standalone
  workflow; remove monorepo-relative paths.
- README.adoc: rewritten for standalone perspective — correct install
  instructions, honest alpha caveats (effects runtime, span fidelity),
  ownership-in-one-minute example, contributing note pointing upstream.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>