feat(runtime): expose post-present refresh batching

[phall1]

Problem

Canvas apps can already batch display-list refreshes internally so platform accessibility publication moves behind the responding present. The Runtime hid that existing seam, forcing a consumer's first installed canvas tree to synchronously publish AppKit accessibility between display-list emission and its first plan/present.

Change

Expose and document the existing begin/end/cancel refresh-batch methods on Runtime. No batching behavior changes. Add a runtime test proving that ending a batch with a frame in flight leaves platform accessibility unpublished until the GPU frame completion flush.

Consumer

Phux Cockpit uses one scoped batch only while installing its main canvas. This removes a measured 170–185ms pre-plan AppKit accessibility publish from first useful frame scheduling; steady-state input batching remains SDK-owned.

Evidence

• branch is based on the merged vertical-splitter Cockpit SDK lineage
• focused runtime test covers deferred publication and post-present flush
• Cockpit consumer gate and M4 Pro live timing run will validate the pinned commit before merge