Describe your changes

Label: separate text and outline passes in SDF shader

• Renamed shader uniform u_effectType => u_labelPass to better reflect its role as a rendering pass selector (0 = text, 1 = outline, 2 = shadow).
• Split text and outline into distinct branches:
  • Text pass draws solid glyph core without outline interference.
  • Outline pass draws only the outer ring, excluding the text core.
  • Shadow pass remains unchanged as pure color fill.
• Introduced u_distanceSpread uniform (default 6.0) for flexible spread control.
• Added outlineScale empirical factor (0.75) to visually match SDF outline thickness
  with non-SDF text rendering.

This change ensures SDF rendering matches non-SDF behavior: text core is never
overwritten by outline, and passes are explicitly separated for clarity.

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