Add rectangular HVAC duct elbows

[gitgrahamdunn]

Summary

• adds the first HVAC duct roadmap slice: rectangular duct elbows
• adds a new DuctElbow parametric object with editable width, height, wall thickness, bend angle, and centerline bend radius
• adds DuctElbow_Rectangular.csv with common metric rectangular duct elbow presets in 45-degree and 90-degree variants
• adds an Insert duct elbow command/form and wires it into the piping toolbar
• marks Add HVAC duct support -> Elbow complete in the roadmap

Source Data

• Rectangular radius elbow convention uses a centerline radius of 1.5x duct width, matching the ductwork design guideline basis in Northwestern 23 3114 - Ductwork: https://www.northwestern.edu/facilities/docs/construction/design_guidelines/23_hvac/NU_23%203114%20-%20DUCTWORK.pdf
• CSV rows store the source URL and keep width, height, wall thickness, bend angle, and bend radius explicit for auditability.

Preview

Validation

• python3 -m py_compile pFeatures.py pCmd.py pForms.py CPipe.py InitGui.py Quetzal_tooltips.py
• git diff --check
• CSV integrity check for 16 rectangular duct elbow rows and 1.5x width radius mapping
• FreeCAD command-mode construction check for direct DuctElbow and pCmd.makeDuctElbow: both produced one solid with nonzero volume and two ports