fix(observation): admit bounded LAB chunk coverage

apps/control-station/test/rerunViewportProgressivePlayback.test.mjs

@@ -42,6 +42,7 @@ test("a first frame reports buffer telemetry but is not ready for presentation",
   const firstFrame = recordedPlaybackBufferState({ min: 0, max: 0 }, 20);
   assert.deepEqual(firstFrame, {
     bufferedEndNs: 0,
+    expectedStartNs: 0,
     expectedEndNs: 20_000_000_000,
     bufferProgress: 0,
     fullyBuffered: false,
@@ -58,6 +59,7 @@ test("host timeline remains unmounted until the verified recording is fully read
     currentNs: 0,
     playing: false,
     bufferedEndNs: 5,
+    expectedStartNs: 0,
     expectedEndNs: 10,
     bufferProgress: 0.5,
     fullyBuffered: false,
@@ -77,6 +79,7 @@ test("host timeline remains unmounted until the verified recording is fully read
 test("buffer progress grows independently and preserves the full-buffer tolerance", () => {
   assert.deepEqual(recordedPlaybackBufferState({ min: 0, max: 5_000_000_000 }, 20), {
     bufferedEndNs: 5_000_000_000,
+    expectedStartNs: 0,
     expectedEndNs: 20_000_000_000,
     bufferProgress: 0.25,
     fullyBuffered: false,
@@ -97,14 +100,37 @@ test("buffer progress grows independently and preserves the full-buffer toleranc
     { min: 0, max: 19_999_500_000 },
   );
 
-  const wrongDeclaredStart = recordedPlaybackBufferState(
+  const missingDeclaredStart = recordedPlaybackBufferState(
     { min: 5_000_000_000, max: 20_000_000_000 },
     20,
     0,
   );
-  assert.equal(wrongDeclaredStart.bufferProgress, 1);
-  assert.equal(wrongDeclaredStart.fullyBuffered, false);
-  assert.equal(isRecordedPlaybackReady(true, true, wrongDeclaredStart), false);
+  assert.equal(missingDeclaredStart.bufferProgress, 1);
+  assert.equal(missingDeclaredStart.fullyBuffered, false);
+  assert.equal(isRecordedPlaybackReady(true, true, missingDeclaredStart), false);
+});
+
+test("a verified split boundary spill covers and clamps the declared LAB window", () => {
+  const actualRange = {
+    min: 34_756_228_209,
+    max: 95_383_857_292,
+  };
+  const buffer = recordedPlaybackBufferState(
+    actualRange,
+    95.383857292,
+    35.421857292,
+  );
+
+  assert.equal(buffer.bufferProgress, 1);
+  assert.equal(buffer.fullyBuffered, true);
+  assert.equal(isRecordedPlaybackReady(true, true, buffer), true);
+  assert.deepEqual(
+    recordedPlaybackRangeWhenReady(actualRange, buffer, true),
+    {
+      min: 35_421_857_292,
+      max: 95_383_857_292,
+    },
+  );
 });
 
 test("recorded autoplay waits for the full range and then runs exactly once", () => {