Feature/fw overflow safety #642

[web3nova]

Overview

This PR mitigates a critical Denial-of-Service (DoS) and incorrect accounting vector within the family_wallet crate by replacing potential panicking/silently-saturating raw integer addition with explicit overflow-safe logic.

Changes

• Arithmetic Safety: Patched validate_precision_spending_internal and record_precision_spending to utilize checked_add with proper error mapping back to Error::InvalidSpendingLimit. This shields accumulation logic involving SpendingTracker and PrecisionSpendingLimit paths from exploitation using hostile boundary amounts.
• Defensive Test Assertions: Added a targeted test variant test_precision_spending_overflow_graceful to family_wallet/src/test.rs ensuring extreme values near i128::MAX trigger graceful errors rather than full transaction abort panics.
• Security Trail: Documented the threat design changes clearly in docs/fw-overflow-safety.md.

Closes #642