A single transaction (e.g. a DEX swap) can produce multiple ERC-20
token transfers. The transaction merger was keyed by tx hash alone,
so only the last token transfer survived. This meant the address-token
view's contract-address filter often matched nothing.
Use a composite key (hash + contract address) so all token transfers
are preserved. Also remove the bare normal-tx entry when it gets
replaced by token transfers to avoid duplicates.
Closes#72
