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Replace Hammer.js with native Pointer Events (#950)

Hammer.js (last released 2016) was a required dependency only for touch
pinch/pan gesture recognition. Reimplement gesture handling on top of the
browser's native Pointer Events in src/gestures.ts, preserving the pan/pinch
behaviour, the pan threshold, the mouse modifier-key gate and the callback
contract (onPanStart/onPanRejected/onZoomStart/...). As Hammer did, the
manager disables browser touch-actions and text selection (user-select) on
the canvas while it is active, and restores the previous values on teardown.

The plugin now has zero runtime dependencies: both hammerjs and
@types/hammerjs are dropped, along with the hammer-simulator/ng-hammerjs test
and docs helpers. Karma gesture specs are driven with synthetic PointerEvents
instead of the Hammer simulator.

At most two pointers take part in a gesture, so a stray third finger can
neither restart nor end a pinch, and a mouse gesture only starts on the
primary button. A second pointer now always ends a pan in progress, whether
or not pinch is enabled: Hammer cancelled its Pan recognizer in that case
but signalled it through pancancel, which the old code never subscribed to,
so state.panning leaked and the chart stopped responding to events. For the
same reason pointercancel aborts a gesture instead of completing it, and
tearing down the manager mid-gesture no longer leaves pan state behind for
the next one to resume.

Co-authored-by: Michel Jung <michel@micheljung.ch>

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