identity: add Modifier.animateContentSizeMotion()

Eases a composable's own height/size changes on the family's M3 Expressive
spatial motion scheme (fastSpatialSpec) instead of jumping — e.g. a text
field that wraps to a new line, or a value swapped for a longer one. Snaps
under reduced motion, like the other identity motion helpers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -30,8 +30,11 @@ are no binary releases; "version" tracks the shared `version` in the root build
   own thin themed `CrashReportActivity`.
 - **`identity`** (Android library) — the family's Material 3 Expressive theme
   factory `FloretExpressiveTheme(lightScheme, darkScheme, …)` (dynamic colour on
-  API 31+, system light/dark fallback, the standard motion scheme) plus
-  `rememberNavSlideSpec()`. Each app passes its own seed-derived colour schemes
+  API 31+, system light/dark fallback, the standard motion scheme) plus the
+  shared motion helpers — `rememberNavSlideSpec()`, the content transitions,
+  `Modifier.predictiveBack()` and `Modifier.animateContentSizeMotion()` (eases a
+  block's own height changes, e.g. a text field wrapping to a new line), each
+  reduced-motion-aware. Each app passes its own seed-derived colour schemes
   and typography — the mechanics are shared, the look is not.
 - **`components`** (Android library) — the shared Compose component vocabulary:
   - `GroupedSurface` / `GroupedRow` + `Position` / `positionOf` — the canonical