feat(vanilla): canvas update transitions on the shared transition clock

Keyed data-update transitions now drive the scatter canvas layer from the
same rAF loop that tweens the SVG axes, trendline and annotations, so a
4,341-point morph reads as one motion instead of two clocks drifting apart.

Points are tweened as a single packed record rather than one per mark:
geometry lives in parallel typed arrays lerped inline, written straight into
the layer's SoA. No DOM, no per-frame allocation. Enters fade in on the
whole-mark `enterAlpha` channel (not `fillOpacity`, which must not drag the
stroke down with it, matching SVG's `fill-opacity`); exits become canvas
ghosts under the live points; survivors interpolate and retarget from a
`snapshot()` on interruption. The snapshot entries stay plain `{type:'point'}`
so an interrupted transition retargets correctly across an svg <-> canvas flip.

Gate 8 now sizes against `max(prev, next)` rather than `next` alone. Exit
ghosts render in the DESTINATION mode, so a 4,341-point canvas chart updating
down to 400 SVG points would otherwise mint ~3,941 SVG ghost circles -- the
exact jank the cap exists to prevent. Canvas mode gets its own default of
20,000 (`CANVAS_DEFAULT_UPDATE_MAX_MARKS`), two orders above the SVG cap
because the per-frame cost model is batched fills, not DOM writes.

Gridline tick-value matching is extracted to a pure `computeGridlineDeltas`
so the canvas path reuses it; the SVG behavior it replaces is pinned first by
a 15-test characterization suite. Tick labels keep their SVG tween and share
the canvas gridlines' clock, so a label never separates from its line.

The from-state pass now ends with a synchronous `canvas.repaint()`: state
writes alone leave the bitmap at the destination, rAF-before-paint is an
accident of ordering, and manual mode has no paint at all between
`beginManualUpdate()` and the first `step()`.

Verified: 4078 tests pass. Gate-8's max rule, the canvas cap, the sync
from-state repaint, the per-step repaint and the exit ghost array were each
mutation-tested and each fails its guard when reverted.

Author: rileyhilliard