[s6pa1rta3n-lab]

Summary of Changes

Addresses #295 by implementing the Time-Weighted Average Reputation (TWAR) Segment Tree for the Soroban registry contract.

Technical Implementation

1. Packed Ledger Storage:

   • Implemented TWARSegmentTree backed by a contiguous 1,024-byte layout in contracts/registry/src/storage.rs.
   • Node storage packed into fixed-size 16-byte slots (weighted_score: i64, anchor_ledger: u32, event_count: u32) to eliminate per-entry overhead and reduce Soroban footprint costs.
   • Slot 0 stores header metadata (magic, half_life_ledgers, last_ledger, total_updates).
   • Persisted via Soroban instance storage as Bytes under TwarStorageKey::TwarTree(Address) with automatic TTL extension.

2. Mathematical Decay Function:

   • Implemented monotonic Q32 fixed-point half-life time-decay function (decay_weight_q32, half_life_decay) without floating point operations.
   • Exact power-of-two halving at integer multiples of half-life, with 3-point quadratic polynomial interpolation for fractional ledger deltas.
   • Symmetrically decays positive credits and negative breach penalties over ledger advances.

3. Segment Tree Aggregation:

   • Supports $O(\log n)$ point updates (update_point), dispute retractions (retract_point), and inclusive range aggregation queries (query_range).
   • Supports $O(1)$ decayed total score evaluation (query_total) and normalized 0..=100 TWAR scoring (twar_score).

4. Gas & Metering Benchmarks:

   • Update execution consumes well under 150,000 Soroban CPU instructions.
   • Verified via unit test test_twar_update_cpu_instruction_budget_under_150k.

5. Exhaustive Unit Tests:

   • Mathematical properties & monotonicity across ledger deltas.
   • Instance storage roundtrip and TTL management.
   • Sequential updates across ring capacity.
   • Multi-year ledger jumps decaying to baseline (50) without overflow or panic.
   • Saturating arithmetic boundaries at i64::MAX and i64::MIN.

Acceptance Criteria Checklist

• Implement TWARSegmentTree struct supporting point updates and range aggregation queries.
• Implement half-life decay calculation based on ledger sequence delta.
• Gas benchmarks demonstrating < 150,000 Soroban CPU instructions per update.
• Unit tests covering edge cases: sequential updates, multi-year ledger jumps, and counter overflow/underflow.

Payout Routing

• EVM (Base/Arbitrum/Polygon/ETH): 0xF46C9F6d70C50BF81ef3588AB523a90a594a2F89
• Stellar: GCL6OXAMLD75BMTINA6EMRUDWK5THQUSHMYNLSNBCJAPZJHNYJTUNIBC

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