Per-widget-type default physics, keyed by platform (D108/Phase 26 Step 2).
This is the ONLY place platform is consulted for scroll behavior — one
pure lookup, never branches scattered through widget code — and it is
always the lowest-priority source: an app’s own theme ext value or an
explicit .physics(...) on a ScrollView both override it. See
rosace-widgets/src/tree/scroll_view.rs’s resolve_physics.