Board + BoardCard: positioned sticky-note cards in board coordinates, immutable
copy-on-write edits (add/removeById/moveCard/setText, unique ids), cardsIn()
broad-phase culling, and linkGraph()/backlinksOf() that derive the 双链 graph
from the cards' [[links]] — making link_graph load-bearing. Cards also host ink
via a StrokeHost keyed by card id (same host-agnostic engine as PDF pages).
Pure model; fully unit-tested (CRUD, immutability, culling, backlinks).
flutter analyze lib/editor clean; 215/215 tests (+7).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
