fix(plex): a seek during a quiet reclaim is no longer undone

A quiet request leaves `live` alone, so a seek inside the old session's
range went native and the answer then jumped back to where it asked from.
`seek` now also goes by `pending`; the server-away retry had the same hole.

Comments that still named 404 for a lost session say 410, and the hls.js
retry comment now tells a finished playlist from a growing one.

frontend/src/lib/hlsEngine.test.ts

@@ -374,7 +374,7 @@ describe("a failed session request", () => {
   });
 });
 
-describe("a session the server no longer has (404)", () => {
+describe("something that never arrives (a fatal 404)", () => {
   it("is reclaimed where the viewer is", async () => {
     const { engine, asks, hlses, state } = setup();
     engine.start("A", 600);
@@ -482,6 +482,17 @@ describe("a session the server says is gone (410)", () => {
     expect(engine.getSnapshot().error).toBe("generic");
   });
 
+  it("keeps a seek made while it is out, inside what the old session still reports", async () => {
+    const { engine, asks, hlses, media } = await playing();
+    media.seekEnd = 800;
+    hlses[0]!.gone();
+    media.currentTime = 42;
+    engine.seek("A", 1250);
+    expect(media.currentTime).toBe(42); // the outgoing media stays where it is
+    await asks[1]!.resolve(hls("A", 1299));
+    expect(asks.map((a) => a.startAt)).toEqual([600, 1299, 1250]);
+  });
+
   it("leaves a segment that is merely not there yet to hls.js's retries", async () => {
     const { engine, asks, hlses } = await playing();
     hlses[0]!.retrying(404);
@@ -493,6 +504,20 @@ describe("a session the server says is gone (410)", () => {
 describe("a server that is restarting (a deploy)", () => {
   // The gateway answers 502 while the app restarts, and nothing at all may answer (status 0): the
   // stream is not broken, the server is away. The buffer keeps playing meanwhile.
+  it("keeps a seek made while the retry is out", async () => {
+    const { engine, asks, hlses, state, fire, media } = setup();
+    engine.start("A", 600);
+    await asks[0]!.resolve(hls("A", 600));
+    hlses[0]!.handlers.onManifestParsed();
+    media.seekEnd = 800;
+    state.position = 1299;
+    hlses[0]!.lost(502);
+    expect(fire()).toBe(true);
+    engine.seek("A", 1250);
+    await asks[1]!.resolve(hls("A", 1299));
+    expect(asks.map((a) => a.startAt)).toEqual([600, 1299, 1250]);
+  });
+
   it.each([502, 503, 504, 0, null])(
     "is waited out on %s: no error, and the stream comes back where the viewer is",
     async (status) => {