Add validated wall height editing

[jonathanmcmichael]

Background

Wall::new validates its geometry, but the authoring model does not yet provide an explicit operation for changing a wall's height and validating the result. This is a small BIM-domain contribution that supports the next authoring slice.

Suggested scope

Add a narrow height-editing method to Wall in crates/rustit-model/src/lib.rs. The method should build and validate the candidate WallGeometry before mutating the wall.

Acceptance criteria

• a positive finite height updates the wall and is visible through Wall::geometry()
• zero, negative, NaN, and infinite heights return the existing geometry/model error
• a rejected edit leaves the previous height unchanged
• focused unit tests cover successful and rejected edits
• cargo fmt --all -- --check, clippy, and workspace tests pass

Out of scope

UI controls, undo/redo, transactions, persistence, and changing every public field in the model.