Finish SCOPE.md (boundary, maturity, deps, non-goals + capability blocks),
rewrite README.md as an extraction-oriented package doc with the initial
src/ module layout (selectors/pdf/highlight) and public API surface.
Selector types stay in citation-engine; selector behavior + viewer
contract owned here (ADR-0006 / SharedContracts §8).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
