fix: never reuse an output address across coinjoin pools

Prevents address reuse in coinjoins across the Electrum plugin, the cli daemon and the web GUI. Invariant: an address (scriptPubKey) appears at most once, in at most one coinjoin, on either side of the transaction, as far as the local client can see.

Within one coinjoin transaction

Reuse case	Plugin	CLI daemon	Web GUI
Two outputs share an address	equal-value and output-count validation aborts the round	register-output rejects an address another peer registered, combine_psbts rejects duplicate output scriptPubKeys	combine_psbts rejects duplicate output scriptPubKeys
Two inputs spend from the same address	finalize_coinjoin rejects duplicate spent-UTXO addresses	combine_psbts rejects two inputs spending UTXOs with the same scriptPubKey	combine_psbts, same check as the daemon
Input spends from an output address	input validation and finalize_coinjoin reject	registration endpoints and combine_psbts reject	register-input and combine_psbts reject

Across coinjoins

Reuse case	Plugin	CLI daemon	Web GUI
Same output address in two pools	fresh address helper skips addresses reserved by active sessions, creator address persisted so resume re-registers the same one	register-output checks all active pools	fresh address from getnewaddress each round
Same address funds inputs of two pools	input validation checks other sessions' pending coins	register-input compares spent-UTXO scriptPubKeys across all active pools	not applicable, one pool per session
Output in one pool, input in another	input validation checks other sessions' output lists	register-output checks other pools' inputs, register-input checks other pools' output addresses	not applicable
Previously used wallet address offered as output	only unused addresses are picked	getreceivedbyaddress rejects addresses that already received funds, covers the user supplied --address path	getnewaddress is always fresh

Closes #43 (closed)