pow.ts: minePow/verifyPow/countLeadingZeroBits. PoW pays NO ONE - it is a free
CPU tax, not a fee. useNostrPublish accepts pow and mines the nonce on the
UNSIGNED event before signing (id is signature-independent -> one signer prompt).
CreateListingPage sends 16 bits per listing (~80ms) and 20 bits when a newcomer
chooses work instead of the 1,000-sat fee (SpamFeePayment.onPowInstead = the free
alternative). parseMarketplaceItem exposes powDifficulty; the card shows a PoW
badge at >=16.
