Implement horizontal swipe policy enforcer

Horizontal swipe detection without live widget pumping is most reliably achieved via the registration pattern: swipe-using widgets register themselves at startup (or the team adds a lint rule / static analysis check). If widget introspection is chosen, call the route builder with a mock context and walk the tree checking widget runtimeType. The workshop document (likeperson.md, section 1.2) explicitly states 'tilbakeknapp fremfor sidelengs-sveip' and 'vertikal scroll er normen' — this is a hard accessibility requirement driven by motor and cognitive accessibility needs of NHF, Blindeforbundet, and HLF users. Note that the StatefulShellRoute bottom tab bar uses horizontal tab switching internally in Flutter — this should be explicitly excluded from the check via a known-exempt widget registry to prevent false positives on the standard tab navigation pattern.

Unit tests using flutter_test. Scenarios: (1) route with GestureDetector(onHorizontalDragEnd) → one violation, (2) route with PageView(scrollDirection: Axis.horizontal) → one violation, (3) route with Dismissible(direction: DismissDirection.horizontal) → one violation, (4) route with PageView(scrollDirection: Axis.vertical) → zero violations, (5) route with GestureDetector(onVerticalDragEnd only) → zero violations, (6) route with no gesture detectors → zero violations, (7) CupertinoPageRoute usage → one violation, (8) multiple routes with mixed swipe usage → correct violation count per route. Test that violation descriptions identify the specific widget type found (PageView vs GestureDetector vs CupertinoPageRoute).