fix: validate receipt/log membership against block txs

[jxom]

Audit Findings Fixed

Two High severity findings:

1. Unverified eth_getBlockReceipts Data Allows Malicious RPC to Inject Orphan Logs/Receipts

A malicious or buggy RPC could return receipts with transaction_hash values not present in the block's transaction list. These orphan receipts/logs would be written to the database unchecked.

Fix: After fetching both blocks and receipts, validate_receipts() verifies that every receipt's transaction_hash exists in the corresponding block's transaction list.

2. Partial eth_getBlockReceipts Response Bypasses Receipt Backfill

If the RPC returns fewer receipts than transactions in a block, sync would succeed but leave permanent gaps in receipt/log data (gas_used, fee_payer remain NULL).

Fix: validate_receipts() verifies that the receipt count matches the transaction count for each block, erroring on mismatch.

Changes

• Added validate_receipts() function in src/sync/decoder.rs that checks both invariants
• Called it from all three sync entry points:
  • SyncEngine::fetch_range (realtime sync)
  • SyncEngine::sync_block (single block sync)
  • sync_range_standalone (gap-fill workers)
• Added 6 unit tests covering happy path, empty blocks, orphan detection, count mismatches, and batch length mismatches

Testing

cargo test --lib decoder::tests  # 11 tests pass (6 new)
cargo test --lib                 # 179 tests pass
cargo check                      # clean (no new warnings)