Plotting: geometry-aware auto weights for Stacked/Paired

[ocots]

Summary

Geometry-aware :auto weights for Stacked and Paired combinators.

Changes

• Stacked: weights=:auto now uses grid_shape(child)[1] (number of rows) instead of leaf count
• Paired: weights=:auto now uses grid_shape(child)[2] (number of columns) instead of leaf count
• This keeps cell heights uniform when a paired block is nested inside a stack (e.g., Stacked(Paired(state, costate), control, …) gets row weights n : l : … rather than 2n : l : …)
• Updated tests to verify the discriminating cases
• Bumped version to 0.27.1-beta

Context

Needed by the CTModels case layer (Phase 3). CTFlows is unaffected.