Ratatui-style constraint layout solver.

Splits a region along a direction (:vertical splits height into rows, :horizontal splits width into cols) according to a list of constraints:

• {:length, n} — exact n cells
• {:percentage, p} — p% of the available space (rounded down)
• {:min, n} — at least n cells (v0.2: behaves as :length)
• {:max, n} — at most n cells (v0.2: behaves as :length)
• {:fill, weight} — distributes remaining space proportional to weight

Apps typically don't call this directly — vbox/1 and hbox/1 from Harlock.Elements take a :constraints opt and the renderer invokes the solver internally. This module exists in the public surface so the constraint shapes are documented and stable.

When the constraints exceed the available space, sizes are truncated from the tail-most non-fill constraints first, and a warning is logged. The solver never crashes on over-constrained layouts.

Round-off from percentages and fill divisions is absorbed by the last fill constraint (or by the last constraint if there are no fills), so the returned sizes always sum exactly to the requested total.