Fix fractions when promoting containers

[pablocpas]

Fixes #9248

Summary

When a directional move promotes a container out of its immediate parent, Sway currently preserves the moved container's fractions and invalidates the ancestor's fractions.

The moved container's fractions were calculated relative to its former parent, while the ancestor remains in the same parent. This differs from i3 and from Sway's other directional reparenting path.

Reset the moved container's geometry and fractions before inserting it, and preserve the ancestor's fractions. Resetting before workspace_insert_tiling() also ensures that a wrapper introduced by workspace_layout cannot inherit the stale fraction.

Test plan

Create this layout with equal fractions:

H[X Y H[A W]]

Focus W and run:

swaymsg move right

On master, moving through the outer workspace edge produces approximately:

 X = 3/14 ≈ 21.43%
 Y = 3/14 ≈ 21.43%
 A = 1/4  = 25%
 W = 9/28 ≈ 32.14%

With this patch, all four top-level entries receive:

1/4, 1/4, 1/4, 1/4

This matches i3 and the existing Sway behavior when W is moved toward the other top-level windows instead of through the outer edge.

Also verify:

• horizontal and vertical promotion;
• promotion through both workspace edges;
• unequally resized top-level siblings retain their relative proportions;
• moving into a neighboring container remains unchanged.

Tests

• meson setup build --buildtype=debug -Dman-pages=disabled -Dswaybar=false -Dswaynag=false -Dtray=disabled -Dgdk-pixbuf=disabled
• ninja -C build