test(sidecar): cover pollUntilStable and structuralHash

Verify the poll returns on two equal snapshots, after transient
churn, and at the cap when never stable; assert the hash ignores
bounds-only flicker and detects content changes.

sidecar/src/test/kotlin/dev/sanderling/sidecar/StabilityPollTest.kt

@@ -0,0 +1,73 @@
+package dev.sanderling.sidecar
+
+import org.junit.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+class StabilityPollTest {
+    @Test fun returnsAfterTwoIdenticalSnapshots() {
+        val snapshots = mutableListOf<String>()
+        val start = System.currentTimeMillis()
+        pollUntilStable(800L) {
+            val value = "stable"
+            snapshots.add(value)
+            value
+        }
+        val elapsed = System.currentTimeMillis() - start
+        assertTrue(elapsed < 600L, "should settle within a couple poll intervals, took ${elapsed}ms")
+        assertTrue(snapshots.size >= 2, "must observe at least two snapshots to confirm stability")
+    }
+
+    @Test fun returnsAfterChangingThenStabilizing() {
+        var calls = 0
+        pollUntilStable(2000L) {
+            calls++
+            if (calls <= 2) "frame-$calls" else "stable"
+        }
+        // After 2 changing frames, calls 3 and 4 are both "stable" -> exit.
+        assertTrue(calls >= 4, "expected at least 4 snapshots, got $calls")
+    }
+
+    @Test fun stopsAtDeadlineWhenNeverStable() {
+        var calls = 0
+        val budget = 240L
+        val start = System.currentTimeMillis()
+        pollUntilStable(budget) {
+            calls++
+            "frame-$calls"
+        }
+        val elapsed = System.currentTimeMillis() - start
+        assertTrue(elapsed in budget..(budget + 200L), "expected to hit cap, elapsed=$elapsed")
+    }
+
+    @Test fun zeroBudgetReturnsImmediately() {
+        var calls = 0
+        pollUntilStable(0L) {
+            calls++
+            "x"
+        }
+        assertEquals(0, calls)
+    }
+
+    @Test fun structuralHashIgnoresBoundsAndIdenticalForSemanticallyEqualTrees() {
+        val a = """
+        {"attributes":{"resource-id":"LoginScreen","bounds":"[0,0,1080,2340]"},
+         "children":[
+           {"attributes":{"resource-id":"LoginEmail","bounds":"[10,10,1070,100]","text":"a@b"},"children":[]}
+         ]}
+        """.trimIndent()
+        val b = """
+        {"attributes":{"resource-id":"LoginScreen","bounds":"[0,0,1080,2342]"},
+         "children":[
+           {"attributes":{"resource-id":"LoginEmail","bounds":"[10,11,1070,101]","text":"a@b"},"children":[]}
+         ]}
+        """.trimIndent()
+        assertEquals(structuralHash(a), structuralHash(b), "bounds-only flicker must not change hash")
+    }
+
+    @Test fun structuralHashDiffersWhenContentChanges() {
+        val a = """{"attributes":{"resource-id":"LoginEmail","text":"a@b"},"children":[]}"""
+        val b = """{"attributes":{"resource-id":"LoginEmail","text":"c@d"},"children":[]}"""
+        assertTrue(structuralHash(a) != structuralHash(b), "text change must alter hash")
+    }
+}