v0.7.2 — merge everywhere, checks in the app

• Board fine-grained merge (ADR-0016 complete): components by ref,
  holes by name, net classes by class; tracks/vias/zones merge as
  content-identity element sets — a moved track is remove+add and merges
  cleanly, so branches routing different nets combine without conflict.
  Every design kind now merges at its natural semantic grain.
• Viewer checks panel: /api/checks runs the live suite per kind
  (validate/DRC/connectivity, ERC/envelopes, the PCBA gate, assembly
  interference, geometry validity) — a checks tab with a green ✓ or a
  violation count, refreshed on live reload.
• In-viewer review mode: gitcad-view --review main adds a review
  tab — gate badge, introduced/fixed per file, side-by-side base/head
  renders. The PR review, in the app.
• gitcad-explore: the branch scoreboard — N design variants judged
  by the same gates (review delta + the executable requirements suite in
  throwaway worktrees), ranked markdown out.

370 tests. Every commit CI-verified green.