[HW] Pre-calibration checks + first calibration → promote v3.1.0

[AldoSeffrin]

Validation gate from the v3.1.0-rc1 tag. Fourth session.

Pre-calibration (mechanical items left open in DEVELOPMENT_PLAN):

• Visual check: no Al 5052-F countersink deformation (assembly was done without torque wrench)
• Set on rigid, flat floor + level via foot bolts (bubble level)
• Rigidity check under load
• (Optional) instrumented re-torque 20–25 N·m if a torque wrench is available

Calibration (DEVELOPMENT_PLAN §2.4):

• Tare routine (empty platform)
• Multi-point calibration 0/20/40/60/80 kg (reference weights verified on a trusted scale beforehand)
• Acceptance: consistent zero-load + known-load error < 0.5%

On pass: tag v3.1.0 + Phase 1 GitHub release (assembly photos + assembly guide).
Depends on: #3

Ref: v3.1.0-rc1 tag message, docs/DEVELOPMENT_PLAN.md §2.4.