feat(scheduler): Implement Agent Ledger, Accounting, and Jain's Fairness Index

[mahaswetha17]

Summary of Changes

This pull request implements the Agent Ledger, Accounting, and Fairness Metrics subsystem for our priority and fair-share scheduling pipeline[cite: 1].

• Agent Record & State (aios/scheduler/ledger.py): Tracks agent_id, priority, weight, arrival_time, wait_ticks, and service_count.
• Waiting & Service Accounting:
  • update_waiting_ticks(): Automatically increments wait time for queued requests to support Navaneedan's priority aging mechanism[cite: 1].
  • record_service(): Logs scheduling service allocations to support Hitesh's stride/fair-share scheduling evaluation[cite: 1].
• Quantitative Observability: Implemented Jain's Fairness Index ($J = (\sum x_i)^2 / (n \times \sum x_i^2)$) to quantitatively measure allocation fairness across workloads[cite: 1].
• Unit Tests (tests/test_ledger.py): Added full test coverage verifying registration, accounting increments, and fairness calculations[cite: 1]. All tests passing locally.

Testing & Verification

• Ran local pytest suite successfully: pytest tests/test_ledger.py -v (all tests passed cleanly).

Thank you for your contribution to AIOS!
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