Replay-Safe SLA Ledger: Deterministic Idempotency + Saturating Monetary Math

[nottherealalanturing]

Summary

This PR applies a minimal hardening patch to make SLA execution replay-safe and payment arithmetic resilient under extreme inputs, while preserving existing behavior for normal flows.

What changed

• Added idempotent replay guard in calculate_sla:
  • repeated submissions for the same outage_id now return the existing latest result
  • no duplicate history append, no duplicate stats mutation, no duplicate event emission
• Hardened arithmetic in result + stats paths:
  • replaced risky multiplication/addition with saturating operations
  • normalized reward division using deterministic Euclidean division
• Fixed audit/view calculation call path argument mismatch to keep deterministic parity and build correctness

Why this closes the assigned issues

• Closes replay/idempotency invariants and repeated-submission history behavior
• Closes sequence/replay safety expectations by making repeated backend submits no-op deterministic
• Closes overflow hardening for penalty/reward/stat aggregation at extreme input ranges
• Closes payout rounding consistency via canonical integer division semantics

Verification

• cargo test could not be run in this environment because Rust/Cargo is not installed here.
• Patch was validated by static inspection against existing replay, event-state, and extreme-value test intent already present in the repository.

Closes #210
Closes #211
Closes #212
Closes #213

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