Rename scale → scale_mode in protocol/struct layer; add control grouping future note

- `Proto_Display_Device_Info.scale` → `scale_mode`
- `Proto_Start_Display.scale` → `scale_mode`
- `PROTO_DISPLAY_CTRL_SCALE` → `PROTO_DISPLAY_CTRL_SCALE_MODE`
- `proto_write_start_display` param and all callers updated
- `on_display` callback param and all sites updated
- `Display_Slot.scale` → `scale_mode` in node
- Control name "Scale" → "Scale Mode"
- planning.md: add control grouping deferred decision

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

dev/cli/controller_cli.c

@@ -163,7 +163,7 @@ static void on_standalone(
 	(*idx)++;
 }
 
-static const char *scale_name(uint8_t s)
+static const char *scale_mode_name(uint8_t s)
 {
 	switch (s) {
 	case 0: return "stretch";
@@ -179,13 +179,13 @@ static void on_display(
 	uint16_t stream_id,
 	int16_t win_x, int16_t win_y,
 	uint16_t win_w, uint16_t win_h,
-	uint8_t scale, uint8_t anchor,
+	uint8_t scale_mode, uint8_t anchor,
 	void *ud)
 {
 	(void)ud;
-	printf("  [%u] display  stream=%u  pos=%d,%d  size=%ux%u  scale=%s  anchor=%s\n",
+	printf("  [%u] display  stream=%u  pos=%d,%d  size=%ux%u  scale_mode=%s  anchor=%s\n",
 		device_id, stream_id, win_x, win_y, win_w, win_h,
-		scale_name(scale),
+		scale_mode_name(scale_mode),
 		anchor == 0 ? "center" : "topleft");
 }