fix: checked arithmetic for RPC data decoding

[jxom]

Summary

Fix four High severity audit findings related to unchecked arithmetic in RPC data decoding.

Changes

1. Timestamp overflow in decode_block — timestamp_secs * 1000 overflows for large timestamps. Now uses checked_mul with i64::try_from validation, returning an error instead of silently wrapping.

2. Log index narrowing in decode_log — log_index and transaction_index were cast with unchecked as i32. Now uses i32::try_from() and returns error on overflow.

3. Receipt index narrowing in decode_receipt — transaction_index was cast with unchecked as i32. Now uses i32::try_from().

4. u64::MAX overflow in tick_realtime — current_to + 1 and tip_num + 1 could overflow when remote_head is u64::MAX. Now uses saturating_add.

All three decode functions now return Result, propagating errors to callers throughout engine.rs.

Tests

• decode_block_timestamp_overflow_returns_error — verifies u64::MAX timestamp returns error
• decode_block_normal_timestamp_succeeds — verifies normal timestamps decode correctly
• decode_log_index_overflow_returns_error — verifies u64::MAX log index returns error
• decode_log_tx_index_overflow_returns_error — verifies u64::MAX tx index returns error
• decode_log_normal_values_succeeds — verifies normal log values decode correctly
• test_saturating_add_prevents_u64_max_overflow — verifies saturating arithmetic at u64::MAX