Persist edits immediately so a fast app-kill loses no state

Reproduced on device: open a file, scroll, long-press-highlight a
word, then recents-wipe the app ~1s after the highlight. The session
file held the file but lost the selection and cursor: the 1s
rate-limit had not elapsed since the previous save, so the final
changes never flushed before the process died (a recents-wipe is a
kill, not a clean shutdown, so Shutdown's final save never runs).

Two changes in saveSessionIfChanged:

- Urgent path: save immediately when the file changes or a selection
  appears/vanishes. These are low-frequency, high-value changes
  (the user just opened a file or highlighted/cleared text), and they
  are exactly what 'where I left off' means.
- sessionSaveInterval 1s -> 250ms: bounds how stale scroll/cursor can
  be at an unlucky kill. The file is a few hundred bytes, so a few
  small writes a second during active scrolling is negligible.

TestRestore_UrgentSaveOnSelection drives the kill race: a selection
made right after the file-open save must land in the saver without
waiting out the interval (fails with the urgent path disabled).

internal/editor/session.go

@@ -30,8 +30,12 @@ import (
 )
 
 // sessionSaveInterval rate-limits the periodic session save. The file is
-// tiny (a few hundred bytes), so one write a second at most is negligible.
-const sessionSaveInterval = time.Second
+// tiny (a few hundred bytes), so a few writes a second at most is
+// negligible. The window bounds how stale the persisted state can be if
+// the process is killed (recents-wipe) right after a change: at 1s, a
+// kill less than a second after the last edit lost the cursor, selection
+// and final scroll. See saveSessionIfChanged for the urgent path.
+const sessionSaveInterval = 250 * time.Millisecond
 
 // SessionState is the relaunch snapshot (see the file doc above). All
 // positions are absolute file byte offsets; Scroll is in Dp. It is a plain
@@ -101,13 +105,21 @@ func (l *Logic) saveSessionIfChanged() {
 		return
 	}
 	now := time.Now()
-	if now.Sub(l.lastSessionSave) < sessionSaveInterval {
-		return
-	}
 	s := l.SnapshotSession()
 	if s == l.lastSession {
 		return
 	}
+	// Save immediately when the file changes or a selection appears or
+	// vanishes — low-frequency, high-value changes (the user just opened a
+	// file or highlighted/cleared text). Everything else (scroll ticks,
+	// cursor moves, selection-handle drags) is high-frequency and stays
+	// rate-limited; a kill within the window then loses at most a fraction
+	// of a second of motion, not the edit state.
+	urgent := s.File != l.lastSession.File ||
+		(s.SelStart >= 0) != (l.lastSession.SelStart >= 0)
+	if !urgent && now.Sub(l.lastSessionSave) < sessionSaveInterval {
+		return
+	}
 	l.sessionSaver(s)
 	l.lastSession = s
 	l.lastSessionSave = now

internal/test/e2e/restore_test.go

@@ -471,3 +471,84 @@ func TestRestore_ShutdownSavesFinalSnapshot(t *testing.T) {
 		}
 	}
 }
+
+// TestRestore_UrgentSaveOnSelection verifies the kill-race fix: a selection
+// appearing (the user just highlighted text) saves immediately, even inside
+// the rate-limit window that the file-open save opened. Without the urgent
+// path, killing the app (recents-wipe) less than a second after highlighting
+// lost the selection and cursor.
+func TestRestore_UrgentSaveOnSelection(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(dir, "a.txt"), []byte("abcdef"), 0644); err != nil {
+		t.Fatal(err)
+	}
+	saves := make(chan editor.SessionState, 16)
+	l := editor.NewLogic(real.NewRealFileSystem(dir), "/", func(string) {})
+	l.SetSessionSaver(func(s editor.SessionState) { saves <- s })
+	go l.Run()
+	defer l.Shutdown()
+
+	// Open the file: the file change is itself urgent, so a save lands as
+	// the load settles. Drain everything up to and including the loaded
+	// snapshot.
+	if _, ok := l.Inspect(func(st *editor.State) any {
+		editor.OpenFile("/a.txt")
+		return nil
+	}); !ok {
+		t.Fatal("Inspect timed out")
+	}
+	loaded := false
+	for i := 0; i < 100 && !loaded; i++ {
+		v, ok := l.Inspect(func(st *editor.State) any {
+			cb := st.Editor.ChunkedBuffer
+			return cb != nil && cb.FileLen() == 6
+		})
+		if ok {
+			loaded = v.(bool)
+		}
+		if loaded {
+			break
+		}
+		time.Sleep(20 * time.Millisecond)
+	}
+	if !loaded {
+		t.Fatal("file did not load")
+	}
+	// Drain saves up to the first one describing the loaded file.
+	deadline := time.After(3 * time.Second)
+	for {
+		select {
+		case s := <-saves:
+			if s.File == "/a.txt" {
+				goto drainDone
+			}
+		case <-deadline:
+			t.Fatal("timed out: no save for the opened file")
+		}
+	}
+drainDone:
+	// Immediately (well inside any rate-limit window after the saves above),
+	// make a selection appear and force a frame: the urgent path must save it
+	// now, not after the interval elapses.
+	if _, ok := l.Inspect(func(st *editor.State) any {
+		st.Editor.SelectionAnchor = 1
+		st.Editor.SelectionStart = 1
+		st.Editor.SelectionEnd = 4
+		st.Editor.CursorPosition = 4
+		st.ScrollOffset = 3
+		return nil
+	}); !ok {
+		t.Fatal("Inspect timed out")
+	}
+	l.ConfigChan() <- editor.ConfigEvent{PixelWidth: 780, PixelHeight: 1688} // force emitFrame
+
+	select {
+	case s := <-saves:
+		if s.SelStart == 1 && s.SelEnd == 4 && s.Cursor == 4 {
+			return
+		}
+		t.Fatalf("unexpected snapshot: %+v", s)
+	case <-time.After(2 * time.Second):
+		t.Fatal("timed out: selection appearance was not saved immediately (urgent path)")
+	}
+}