Add oracle deviation monitoring and circuit breaker reports

[TUPM96]

Summary

• add per-asset oracle incident reports and oracle_incident events for source divergence, stale feeds, volatility trips, and stabilization
• pause oracle reads automatically when fresh sources diverge by >10% or a price moves >20% within 10 minutes; >2% source divergence records an alert without pausing
• require post-cooldown stable observations before clearing the circuit breaker, and expose breaker/report query methods on the contract
• add focused regression tests for source alerts, source-triggered pause, volatility pause, and gradual unpause

Closes #381

Verification

• git diff --check
• cargo check -p hello-world currently fails on existing baseline issues unrelated to this patch, including duplicate cross_asset_borrow, duplicate DataKey, duplicate generated get_reserve_balance, and non-test builds importing soroban_sdk::testutils from src/tests.
• cargo test -p hello-world oracle_circuit_breaker_test -- --nocapture is blocked by the same existing hello-world baseline compile errors before the focused oracle tests can run.

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