Native SDK split-collapse example

The smallest honest pane-collapse animation harness: a two-pane split whose sidebar collapses and expands over 180 ms, built to measure frame pacing during a layout tween and to demonstrate the runtime layout-tween primitive against the manual idiom it replaces.

Three driving modes, selected by environment variable:

Default (runtime tween): the model owns only the resting collapsed flag; the layout_tweens hook declares the split's target fraction and the runtime eases the rendered fraction toward it, one step per presented frame — no per-frame Msgs, and reduced-motion appearances snap.
SPLIT_COLLAPSE_MARKUP=1 (markup tween): the same primitive declared entirely in markup — resize-duration="180" (and resize-easing) on the split element in src/split_collapse.native makes the bound value a tween target, so the view file is the whole animation and no Zig hook exists.
SPLIT_COLLAPSE_MANUAL=1 (manual ticks): the historical idiom — on_frame returns a tick Msg carrying the presented frame's timestamp, update eases the fraction, and every tick is a full rebuild.

Every tween step logs its arrival cadence on stderr (tween-frame dt_ms=... in manual mode, tween-echo fraction=... in the runtime and markup modes), so a driver can count the visible steps of the 180 ms collapse and read the real deltas between frames.

Extra knobs:

SPLIT_COLLAPSE_AUTO_MS=<interval> arms a repeating auto-toggle so the tween runs without automation.
SPLIT_COLLAPSE_WEB=1 snaps a live webview to the content pane, so the tween reflows real web content beside the collapsing sidebar.

Run with the macOS system backend:

native dev

Run the model-contract tests headless:

native test -Dplatform=null