feat(core): the animated-layout module's in-place swap (spec:AST-105 FR8)

.changeset/animated-layout-swap.md

@@ -0,0 +1,7 @@
+---
+'@astryxdesign/core': patch
+---
+
+[fix] The animated-layout module gains its internal in-place swap: old content held as one inert, transient copy while new content enters in its place, on the shared motion. Nothing uses it yet and nothing public changes. In right-to-left containers, content after a Collapsible or TreeList size change (with an `animatedLayout` handle) now glides by its inline-start edge instead of jumping.
+
+@cixzhang

packages/core/src/motion/animatedLayout.boundary.test.ts

@@ -111,6 +111,9 @@ describe('architecture:react-component-runtime INV12 source boundary', () => {
         'playLayoutChange',
         'applyLayoutChangeTransition',
         'createAnimatedLayout',
+        'holdLayoutSwap',
+        'playLayoutSwap',
+        'playLayoutFrames',
       ]) {
         expect(
           source,
@@ -120,6 +123,16 @@ describe('architecture:react-component-runtime INV12 source boundary', () => {
     }
   });
 
+  it("keeps the swap's copy mechanism private to the module", () => {
+    const users = sourceFiles(coreSrc)
+      .filter(path =>
+        /from '[./]+(motion\/)?transientCopy'/.test(readFileSync(path, 'utf8')),
+      )
+      .map(path => relative(coreSrc, path))
+      .sort();
+    expect(users).toEqual(['motion/animatedLayout.ts']);
+  });
+
   it('is used only by the hook and the components that adopt it', () => {
     const users = sourceFiles(coreSrc)
       .filter(path => path !== join(here, 'animatedLayout.ts'))