Add support for the kitty graphics protocol

Kitty-capable terminals (kitty, ghostty, and others) can display
inline images via APC escape sequences.

Kitty's image support uses a passthrough model: APC sequences from
programs running inside panes are relayed verbatim to the outer
terminal with cursor positioning adjusted for the pane offset.

The outer terminal handles all image rendering and therefore itself must
be kitty-aware.

tty.c

@@ -391,6 +391,23 @@ tty_send_requests(struct tty *tty)
 		return;
 
 	if (tty->term->flags & TERM_VT100LIKE) {
+#ifdef ENABLE_KITTY
+		/*
+		 * Send the kitty graphics capability probe BEFORE the DA1
+		 * request.  Per the kitty spec, a supporting terminal sends
+		 * its APC response before processing any subsequent requests.
+		 * So the reply ordering will be:
+		 *   1. ESC _ G i=31;OK ESC \   (kitty response, if supported)
+		 *   2. ESC [ ? ... c            (DA1 response)
+		 *   3. ESC [ > ... c            (DA2 response)
+		 *   4. ESC P > | ... ESC \      (XDA response)
+		 * which is exactly what our parsers expect.
+		 * Only probe if the kitty feature isn't already enabled.
+		 */
+		if (~tty->term->flags & TERM_KITTY)
+			tty_puts(tty,
+			    "\033_Gi=31,s=1,v=1,a=q,t=d,f=24;AAAA\033\\");
+#endif
 		if (~tty->flags & TTY_HAVEDA)
 			tty_puts(tty, "\033[c");
 		if (~tty->flags & TTY_HAVEDA2)
@@ -1468,6 +1485,7 @@ tty_set_client_cb(struct tty_ctx *ttyctx, struct client *c)
 void
 tty_draw_images(struct client *c, struct window_pane *wp, struct screen *s)
 {
+#ifdef ENABLE_SIXEL
 	struct image	*im;
 	struct tty_ctx	 ttyctx;
 
@@ -1491,6 +1509,9 @@ tty_draw_images(struct client *c, struct window_pane *wp, struct screen *s)
 		ttyctx.allow_invisible_panes = 1;
 		tty_write_one(tty_cmd_sixelimage, c, &ttyctx);
 	}
+#else
+	(void)c; (void)wp; (void)s;
+#endif
 }
 #endif
 
@@ -2144,6 +2165,102 @@ tty_cmd_sixelimage(struct tty *tty, const struct tty_ctx *ctx)
 }
 #endif
 
+#ifdef ENABLE_KITTY
+static int
+tty_has_kitty(struct tty *tty)
+{
+	return ((tty->term->flags & TERM_KITTY) ||
+	    tty_term_has(tty->term, TTYC_KTY));
+}
+
+/*
+ * Pass a kitty APC sequence directly to all attached kitty-capable clients
+ * showing the given pane.  The outer terminal's cursor is first moved to
+ * the pane-relative position (cx, cy) so that images placed at "current
+ * cursor" land in the right spot.  Pass cx=cy=UINT_MAX to skip cursor
+ * positioning (e.g. for delete commands that don't depend on position).
+ */
+void
+tty_kitty_passthrough(struct window_pane *wp, const char *data, size_t len,
+    u_int cx, u_int cy)
+{
+	struct client	*c;
+	struct tty	*tty;
+	u_int		 x, y;
+
+	TAILQ_FOREACH(c, &clients, entry) {
+		if (c->session == NULL || c->tty.term == NULL)
+			continue;
+		if (c->flags & CLIENT_SUSPENDED)
+			continue;
+		if (c->tty.flags & TTY_FREEZE)
+			continue;
+		tty = &c->tty;
+		if (!tty_has_kitty(tty))
+			continue;
+		if (c->session->curw->window != wp->window)
+			continue;
+		if (!window_pane_visible(wp))
+			continue;
+
+		/* Position cursor at the correct screen location. */
+		if (cx != UINT_MAX && cy != UINT_MAX) {
+			x = wp->xoff + cx;
+			y = wp->yoff + cy;
+			if (status_at_line(c) == 0)
+				y += status_line_size(c);
+			tty_region_off(tty);
+			tty_margin_off(tty);
+			tty_cursor(tty, x, y);
+		}
+
+		tty->flags |= TTY_NOBLOCK;
+		tty_add(tty, data, len);
+		tty_invalidate(tty);
+	}
+}
+
+/*
+ * Delete all kitty image placements from the outer terminal unconditionally.
+ * Called directly (not via tty_write) so it fires on every full window
+ * redraw regardless of whether the current window has any stored images.
+ */
+void
+tty_kitty_delete_all(struct tty *tty)
+{
+	char	*data;
+	size_t	 size;
+
+	if (!tty_has_kitty(tty))
+		return;
+
+	if ((data = kitty_delete_all(&size)) == NULL)
+		return;
+
+	tty->flags |= TTY_NOBLOCK;
+	tty_add(tty, data, size);
+	tty_invalidate(tty);
+	free(data);
+}
+
+/*
+ * Delete all kitty image placements via passthrough for a specific pane.
+ * Used on terminal reset (RIS) so images are cleared from the outer terminal.
+ */
+void
+tty_kitty_delete_all_pane(struct window_pane *wp)
+{
+	char	*data;
+	size_t	 size;
+
+	if ((data = kitty_delete_all(&size)) == NULL)
+		return;
+
+	tty_kitty_passthrough(wp, data, size, UINT_MAX, UINT_MAX);
+	free(data);
+}
+#endif
+
 void
 tty_cmd_syncstart(struct tty *tty, const struct tty_ctx *ctx)
 {