feat: migration to rewards by timestamp

[noa-starkware]

Note

Medium Risk
Touches staking reward configuration and initialization logic, which can affect on-chain reward calculations and epoch transitions if the new initialization path is incorrect. Test updates reduce risk but coverage is still limited to the exercised scenarios.

Overview
Consensus-rewards migration now initializes timestamp-based block rewards on configuration. When set_consensus_rewards_first_epoch is called for the first time, the staking contract precomputes and stores block_rewards (and updates last_calculated_epoch) to avoid starting consensus rewards without initialized per-block rewards.

Tests and flows are updated to reflect timestamp-based progression: several staking tests advance an epoch before configuring consensus rewards, the set_consensus_rewards_first_epoch test now asserts block_rewards/last_calculated_epoch initialization, and flow tests replace advance_block_number_global with advance_blocks(..., block_duration) to advance time as well as block number. Documentation/notes add additional migration test ideas for rewards-by-timestamp.

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This change is 

[remollemo]

@remollemo reviewed 3 files and all commit messages, and made 1 comment.
Reviewable status: 3 of 4 files reviewed, all discussions resolved (waiting on NirLevi-starkware).

[codecov]

Codecov Report

❌ Patch coverage is 98.11321% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.82%. Comparing base (2de7a34) to head (e017847).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/staking/staking.cairo	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
- Coverage   95.83%   95.82%   -0.01%     
==========================================
  Files          42       42              
  Lines       10385    10434      +49     
==========================================
+ Hits         9952     9998      +46     
- Misses        433      436       +3     

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