Move the pinch-zoom (pinchZoom) and inertia-step (stepInertia) geometry out of
the panZoomGestures DOM glue into pure, unit-tested helpers in panZoom.ts, with
named FRAME_MS/INERTIA_* constants. Addresses the QA blocker that the gesture
module's core math was untested. No behaviour change.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Add a panZoomGestures action: one-finger/left-button drag pans, two-finger
pinch and Ctrl+wheel zoom around the centroid, plain wheel pans. Pan is
edge-clamped via clampPan (no infinite scroll), a real drag suppresses the
trailing node click, and inertia decays after release unless prefers-reduced-
motion. Canvas container switches from native scroll to overflow-hidden.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
