Add runtime text shadow support

docs/protocol.md

@@ -44,6 +44,16 @@ Final opacity = color alpha × node alpha × runtime animation alpha
 
 `#RRGGBB` colors behave as fully opaque colors. `#00000000` is fully transparent.
 
+### Text shadow
+
+Text-capable nodes may use flat shadow fields:
+
+- `textShadowColor`: `#RRGGBB` or `#AARRGGBB` shadow color.
+- `textShadowOffsetX` / `textShadowOffsetY`: shadow offset in runtime pixels.
+- `textShadowBlur`: non-negative blur radius.
+
+The shadow color alpha is multiplied by `RuntimeNode.alpha` and any runtime animation alpha.
+
 ## RuntimeCommand
 
 Runtime commands request generic side effects owned by Dart/Flame.