identity: add the shared content transitions

expandEnter/collapseExit (revealable sections), itemEnter (resolved content),
animateItemMotion (lazy-list relayout) and fadeThrough — all driven by the M3
Expressive motion scheme and honouring rememberReduceMotion. Drawn from
Calendula's motion helpers.

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

identity/build.gradle.kts

@@ -36,6 +36,7 @@ dependencies {
     implementation(libs.androidx.ui)
     implementation(libs.androidx.foundation)
     implementation(libs.androidx.material3)
+    implementation(libs.androidx.animation)
     // activity-compose for PredictiveBackHandler + BackEventCompat (the back-gesture peek).
     implementation(libs.androidx.activity.compose)
 }

identity/src/main/kotlin/de/jeanlucmakiola/floret/identity/Transitions.kt

@@ -0,0 +1,91 @@
+package de.jeanlucmakiola.floret.identity
+
+import androidx.compose.animation.ContentTransform
+import androidx.compose.animation.EnterTransition
+import androidx.compose.animation.ExitTransition
+import androidx.compose.animation.expandVertically
+import androidx.compose.animation.fadeIn
+import androidx.compose.animation.fadeOut
+import androidx.compose.animation.shrinkVertically
+import androidx.compose.animation.slideInVertically
+import androidx.compose.animation.togetherWith
+import androidx.compose.foundation.lazy.LazyItemScope
+import androidx.compose.material3.ExperimentalMaterial3ExpressiveApi
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Modifier
+
+/**
+ * The family's content transitions, driven by the active M3 Expressive motion
+ * scheme so every app reveals, collapses and re-lays-out the same way. Each
+ * honours [rememberReduceMotion]: when the user has asked the system to remove
+ * animations, the spatial movement is dropped and only a quick fade remains.
+ *
+ * Drawn from Calendula's motion helpers; pair [expandEnter] with [collapseExit]
+ * on an `AnimatedVisibility`, use [itemEnter] for content revealed once it
+ * resolves, [animateItemMotion] for `LazyColumn`/`LazyRow` row relayout, and
+ * [fadeThrough] to swap whole blocks with no spatial direction.
+ */
+
+/**
+ * Enter for a vertically-revealed section (expandable rows, inline fields):
+ * height grows from the top while fading in; a quick fade only under reduced
+ * motion. Pair with [collapseExit].
+ */
+@OptIn(ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+fun expandEnter(reduceMotion: Boolean = rememberReduceMotion()): EnterTransition {
+    val fade = fadeIn(MaterialTheme.motionScheme.fastEffectsSpec())
+    return if (reduceMotion) fade else expandVertically(MaterialTheme.motionScheme.fastSpatialSpec()) + fade
+}
+
+/** Exit counterpart to [expandEnter]: shrink + fade, or fade only under reduced motion. */
+@OptIn(ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+fun collapseExit(reduceMotion: Boolean = rememberReduceMotion()): ExitTransition {
+    val fade = fadeOut(MaterialTheme.motionScheme.fastEffectsSpec())
+    return if (reduceMotion) fade else shrinkVertically(MaterialTheme.motionScheme.fastSpatialSpec()) + fade
+}
+
+/**
+ * Enter for content revealed by an `AnimatedContent`/`AnimatedVisibility` (e.g.
+ * results once a query resolves): a gentle rise + fade; fade only under reduced
+ * motion.
+ */
+@OptIn(ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+fun itemEnter(reduceMotion: Boolean = rememberReduceMotion()): EnterTransition {
+    val fade = fadeIn(MaterialTheme.motionScheme.fastEffectsSpec())
+    return if (reduceMotion) fade else fade + slideInVertically(MaterialTheme.motionScheme.fastSpatialSpec()) { h -> h / 6 }
+}
+
+/**
+ * `LazyItemScope.animateItem` wired to the app's motion scheme instead of
+ * Compose's default spring, so list rows fade/relocate consistently. Returns a
+ * bare [Modifier] under reduced motion so rows snap into place. Requires stable
+ * item keys.
+ */
+@OptIn(ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+fun LazyItemScope.animateItemMotion(reduceMotion: Boolean = rememberReduceMotion()): Modifier =
+    if (reduceMotion) {
+        Modifier
+    } else {
+        Modifier.animateItem(
+            fadeInSpec = MaterialTheme.motionScheme.fastEffectsSpec(),
+            placementSpec = MaterialTheme.motionScheme.fastSpatialSpec(),
+            fadeOutSpec = MaterialTheme.motionScheme.fastEffectsSpec(),
+        )
+    }
+
+/**
+ * Cross-fade [ContentTransform] for swapping whole screens or blocks with no
+ * meaningful spatial direction (e.g. onboarding gates). Pure opacity, so it
+ * doubles as its own reduced-motion form.
+ */
+@OptIn(ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+fun fadeThrough(): ContentTransform {
+    val fade = MaterialTheme.motionScheme.fastEffectsSpec<Float>()
+    return fadeIn(fade).togetherWith(fadeOut(fade))
+}