feat(canvas): support vertical split axes

[phall1]

Change

Generalize the existing native canvas splitter across horizontal and vertical axes while preserving horizontal as the source-compatible default. Pointer drag, keyboard steps, focus, adjustable accessibility actions, cursor routing, optimistic layout echo, paint, and min-size clamping all use the declared axis.

SplitAxis is packed into the existing WidgetRuntimeFlags byte, preserving the retained hot-path Widget size at 776 bytes.

Consumer

Phux Cockpit uses this on its pinned cockpit/v0.9.5 lineage to give Split Down the same divider behavior as Split Right. This branch is a direct descendant of 7e783da8, the existing Cockpit pin.

Evidence

• root SDK suite compiled all changed widget/runtime/platform paths; the new vertical splitter test and retained Widget size tests passed
• final root run reached 520/522 steps and 3341/3358 tests; only three pre-existing timing-sensitive effects teardown tests failed under machine load
• earlier full fast gate also exposed missing local TypeScript/Android toolchains unrelated to this axis change
• Cockpit consumer gate and live automation will run against the published pin before merge