Preserve absolute frame coordinates on secondary displays

[TechDufus]

What changed

This preserves absolute frame coordinates when GridLayout applies frame-based placements through hs.layout.apply().

• add options.absolute_x = true and options.absolute_y = true to normalized layout rows in helpers.lua
• extend tests/helpers_spec.lua to assert those flags for legacy and screen-aware cells

Why

A display positioned to the left or above the primary display uses negative global coordinates. GridLayout computes the correct absolute frame for that screen, but hs.layout.apply() treats negative frame.x and frame.y values as offsets unless the row opts into absolute coordinates.

That caused layout changes on a secondary display to pull windows back onto the primary display instead of applying the frame on the intended screen.

Impact

Screen-aware and legacy frame-based placements now preserve the real global coordinates of the target display, so layouts continue to apply correctly on non-primary displays with negative origins.

Validation

• lua tests/helpers_spec.lua
• luac -p helpers.lua grid.lua tests/helpers_spec.lua

[TechDufus]

Superseded by #2. #1 was opened from an existing feature branch and included unrelated screen-aware-cells history.