[CPritch]

The shader zooms in 2.5% on each side, so shifted pixels come from beyond the screen instead of the stretched edge. That's enough for a parallax_intensity up to 0.05. Past that, the biggest shifts reached the edge.

• The margin is now 2.5% or half the intensity, whichever is more, so the default and anything up to 0.05 look the same as before
• The shader works it out from the intensity it already gets. On the Rust side, start_transition zooms the scale it hands the portal and dissolve by the same margin, from the intensity step_and_write last drew with
• The config docs no longer say the edges show past 0.05

Nearfar wants a stronger parallax on phones (0.08), which is what this is for. Format, clippy and tests pass locally, with a new test that the margin covers the biggest shift, and the render crate still checks for Android. Not yet run on the desktop.