fix(app): make session navigation stable and fast (#33569)

Co-authored-by: Brendan Allan <git@brendonovich.dev>

packages/app/e2e/performance/unit/first-navigation-metrics.test.ts

@@ -0,0 +1,32 @@
+import { expect, test } from "bun:test"
+import { summarizeFirstNavigation } from "../timeline/first-navigation-metrics"
+
+test("reports blank frames before first destination and stable paint", () => {
+  expect(
+    summarizeFirstNavigation([
+      { observedAtMs: 16, source: true, destination: false, content: true },
+      { observedAtMs: 32, source: false, destination: false, content: false },
+      { observedAtMs: 48, source: false, destination: true, content: true },
+      { observedAtMs: 64, source: false, destination: true, content: true },
+      { observedAtMs: 80, source: false, destination: true, content: true },
+    ]),
+  ).toEqual({
+    samples: 5,
+    firstDestinationObservedMs: 48,
+    stableDestinationObservedMs: 80,
+    sourceSamples: 1,
+    blankSamples: 1,
+    unknownSamples: 0,
+    destinationSamples: 3,
+  })
+})
+
+test("does not report stability for interrupted destination frames", () => {
+  expect(
+    summarizeFirstNavigation([
+      { observedAtMs: 16, source: false, destination: true, content: true },
+      { observedAtMs: 32, source: false, destination: false, content: true },
+      { observedAtMs: 48, source: false, destination: true, content: true },
+    ]).stableDestinationObservedMs,
+  ).toBeNull()
+})