HyprTile F11 Wallpaper Pipeline - Stage Blend Modes
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Renderer
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- Added true per-pack stage blend modes to hyprtile-wallpaperd.
- New CLI: --pack-blend normal|screen|add|multiply|overlay|softlight|difference
- Blend happens once after a complete pack, so multi-pass presets remain one logical stage.
- Stage opacity is applied after the blend function:
    output = mix(previous, blend(previous, stage), opacity)
- Internal manifest pass opacity remains normal pass-local mixing.
- normal + 100% keeps the direct fast path with no extra stage composite.
- Non-normal modes preserve the pre-stage image and use the internal compositor.

Persistent model
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- wallpaper_shader_chain items now persist "blend".
- Missing/empty/unknown values normalize safely to "normal".
- Accepted aliases include additive, mul, soft-light, soft_light and diff.
- Existing enabled/opacity/text/font behavior is unchanged.

F11
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- Stage Edit now exposes Blend before Opacity.
- Previous/next blend controls plus one-click Normal reset.
- Context description explains the purpose of each mode.
- Compact pipeline rows show non-default "Stage mix: <mode> · <opacity>" metadata.
- Preview, Revert and Apply All use the exact same persisted blend model.

Supported modes
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normal, screen, add, multiply, overlay, softlight, difference
