feat: calculate block rewards by timestamp

[noa-starkware]

This change is 

---

Note

Switches to timestamp-based per-epoch block rewards (cached) and updates tests/utilities, including reward supplier V3 upgrade and block-time-aware helpers.

• Staking (core):
  • Add last_calculated_epoch and block_rewards to Storage and compute block rewards via calculate_block_rewards(...) using per-epoch caching and update_current_epoch_block_rewards().
  • Wire new calc into update_rewards (pass curr_epoch).
• Test utilities:
  • Introduce calculate_current_block_rewards_v3(...) and variant with custom avg block time; derive rewards from yearly mint and block duration (SECONDS_IN_YEAR, BLOCK_DURATION_SCALE).
  • Add advance_blocks(blocks, block_duration) to move block number and timestamp together.
• Flows/Tests:
  • Replace advance_block_number_global with advance_blocks(..., AVG_BLOCK_DURATION) where rewards are updated per block; adjust expectations to block-level rewards helpers and drop unused staking_contract params.
  • Use new block-reward helpers in staking tests; ensure consistent rewards when blocks are missed and across epochs.
• Upgrade path:
  • Extend V3 system upgrade to also upgrade RewardSupplier implementation.
• Docs:
  • Add "block rewards by timestamp" test ideas to flow_test/flow_ideas.md.

^{Written by Cursor Bugbot for commit 313688c. This will update automatically on new commits. Configure here.}