feat: implement warmup period for grants with linear scaling from 25%…

[Henrichy]

closes #41

• Add warmup_duration and start_time fields to Grant struct
• Implement calculate_warmup_multiplier() for linear scaling over warmup period
• Update settle_grant() to apply ramping multiplier during warmup
• Modify create_grant() to accept warmup_duration parameter
• Add write_grant() helper function for state persistence
• Update all existing tests to support new warmup_duration parameter
• Add comprehensive tests for warmup functionality:
  • test_warmup_period_linear_scaling
  • test_no_warmup_period
  • test_warmup_with_withdrawal

[Henrichy]

@JerryIdoko please approve this PR as all criterias have been met