Add support for DECSTR soft screen reset escape code

This escape code is largely undefined. There is no specification for how
it affects alternate screen mode, overriden colors, kitty keyboard
state, paused rendering, etc. Do what I feel is sensible in these cases.

Fixes #10263

kitty_tests/__init__.py

@@ -60,7 +60,7 @@ class Callbacks:
     def notify_child_of_resize(self):
         self.num_of_resize_events += 1
 
-    def on_reset(self) -> None:
+    def on_reset(self, is_hard_reset: bool = True) -> None:
         if self.pty is not None:
             self.pty.reset_termios_state()
 

kitty_tests/screen.py

@@ -1864,6 +1864,57 @@ class TestScreen(BaseTest):
             sc(2, 1, 2, 3, slot=slot)
             sc(5, 13, slot=slot)
 
+    def test_soft_reset(self):
+        SOFT_RESET = b'\x1b[!p'  # DECSTR sequence
+
+        # Screen content is preserved (unlike hard reset)
+        s = self.create_screen()
+        s.draw('hello')
+        parse_bytes(s, SOFT_RESET)
+        self.ae(str(s.line(0)), 'hello')
+
+        # Hard reset clears screen content; soft reset does not
+        s = self.create_screen()
+        s.draw('hello')
+        s.reset()
+        self.ae(str(s.line(0)), '')
+
+        # Cursor SGR attributes are cleared
+        s = self.create_screen()
+        s.select_graphic_rendition(1)   # bold
+        s.select_graphic_rendition(31)  # red fg
+        self.assertTrue(s.cursor.bold)
+        self.assertNotEqual(s.cursor.fg, 0)
+        parse_bytes(s, SOFT_RESET)
+        self.assertFalse(s.cursor.bold)
+        self.ae(s.cursor.fg, 0)
+
+        # Cursor position is preserved
+        s = self.create_screen()
+        s.cursor_position(3, 4)
+        self.ae((s.cursor.y, s.cursor.x), (2, 3))
+        parse_bytes(s, SOFT_RESET)
+        self.ae((s.cursor.y, s.cursor.x), (2, 3))
+
+        # Insert mode (IRM) is cleared
+        s = self.create_screen()
+        s.draw('abcde')
+        s.set_mode(IRM)
+        parse_bytes(s, SOFT_RESET)
+        # without IRM, drawing overwrites
+        s.cursor.x = 0
+        s.draw('X')
+        self.ae(str(s.line(0)), 'Xbcde')
+
+        # Alternate screen is NOT exited on soft reset
+        s = self.create_screen()
+        parse_bytes(s, b'\x1b[?1049h')  # enter alternate screen
+        self.assertFalse(s.is_main_linebuf())
+        parse_bytes(s, SOFT_RESET)
+        self.assertFalse(s.is_main_linebuf())
+        s.reset()
+        self.assertTrue(s.is_main_linebuf())
+
 
 def detect_url(self, scale=1):
     s = self.create_screen(cols=30 * scale)