fix(balance-sizes): preserve total weight instead of resetting to a constant

[MsfPablo]

Summary

Fixes #1837. balance(_:) assigned a hardcoded weight of 1 to every child, discarding whatever total the container's weights previously summed to.

Weights are only converted to real pixel sizes by the layout pass, which runs once after a whole binding's command list finishes. So when balance-sizes and resize are chained in the same binding, resize's delta calculation still operates against the pre-existing (larger) weight scale of sibling containers, while the just-balanced children were reset to a tiny constant (1) — producing a much wider window than running the same two commands as separate keybind presses.

Fix

Redistribute the container's existing weight total evenly across its children (total / children.count) instead of assigning an arbitrary constant, so later commands in the same list see a consistent scale.

Test plan

• Updated testBalanceSizes (total 1+2+3=6 → each child becomes 2, not the old hardcoded 1)
• Added testBalanceSizes_nestedContainerPreservesItsOwnTotal, confirming each nested container's total is redistributed independently
• Added testBalanceSizesThenResize_behavesLikeResizeAlone, the regression test for the exact chained-command scenario from the issue

I couldn't run the test suite itself in my environment — swift test fails with error: no such module 'XCTest', which I confirmed is pre-existing on a clean main checkout with no changes at all (I only have Xcode Command Line Tools, not a full Xcode install). swift build (the actual app code) compiles cleanly with this change. Happy to have CI confirm the test suite.

[MsfPablo]

Ping on this. The current balance-sizes resets total weight to a constant, which shifts the container's overall size; this preserves it. Happy to walk through the math if useful.