Add Franka CELLO variant: non-decoupling ring brakes, base bearing, 3D-printed trigger spring

[pilcsimo]

Summary
This PR contributes CELLO: low-cost GELLO leader variant for the Franka Emika Panda, that improves joint stability via frictional joint regularization and offloads structure-borne loads from motors.

Motivation
The Franka GELLO relies on springs/rubber bands that were hard to source in the EU and degraded in use, leading to joint sag and fragile handling. We replace spring regularizers with compact, adjustable ring brakes and add a concentric base bearing to isolate radial/axial loads. Encoder fidelity is preserved (no slip clutch in series).

What’s included

• Non-decoupling ring brakes (axial + radial variants, parametric). TPU pads (≈ Shore 95A) with symmetric M3×25 preload → adjustable drag so links “stay put.”
• Bearing-isolated base joint: motor transmits torque; bearing carries radial/axial loads.
• Printable trigger torsion spring: biases trigger open; simple replacement.
• Tightened tolerances, redesigned for better printability: reinforced interfaces and better-tailored to modern printers.
• Docs & assets: README, assembly guide with photos & BOM, CAD (STEP), STLs, 3MF projects (orientations + painted supports).

Validation

• more than a month usage: no visible wear on printed spring; improved rigidity and operational stability.
• Trade-offs: modest increases in mass, mechanical complexity, assembly time, screw accessibility.

Compatibility
Designed for Franka Panda geometry; tested with same motors as the Franka GELLO. Brakes are parametric and can be adapted to other GELLO variants.

License / attribution
Credits and license align with upstream GELLO. Load-path choices informed by FACTR.