Upward / Downward Passes and Solver

Upward Pass: `upward_measure_pass_cached`

walks from the deepest level back to the root
computes IntrinsicSize for containers from their children
ignores Absolute items that are out of flow
uses the cache to skip clean nodes

Downward Pass: `downward_solve_pass_safe`

walks from the root toward deeper levels
builds SolverConfig and SolverSpec for each node
calls solve_flex_layout
writes results into:
- ComputedSize
- Transform

Solver

In src/layout/core/solver.rs, the solver:

separates in-flow and absolute items
resolves main and cross sizes
applies flex grow and shrink
applies align and stretch rules
resolves the absolute box at the end

`translate_spec` / `translate_config`

translate_config reads container-level data from ULayout
translate_spec reads item-level data from USelf

This translation layer is the bridge between public layout components and the internal solver.