[dhyabi2]

Problem
A merchant whose 402 offers only x402 exact on nano:mainnet can't be paid with pay today: there's no Nano keystore, no scheme registered for that network, and quotes lists the rail as unsupported.

Change

• src/chains/nano.ts: ed25519-blake2b keys, SLIP-10 m/44'/165'/0' for mnemonics, checksummed nano_/xrb_ addresses, exact raw/XNO conversion with bigint, a balance read, and a signed state send block published through a swappable RPC (work_generate + process); NANO_RPC_URL overrides the endpoint.
• src/chains/nano-x402.ts: ExactNanoScheme, registered on nano:mainnet in payViaX402 next to Base. Payload { blockHash }. A retry within one invocation re-sends the block already published instead of paying again.
• Nano is opt-in: used only with --chain nano; never auto-selected, not in the default wallet create, never priced in USD.
• pay --chain nano requires --max-spend-xno (else limit_required), aborts over the cap before any block is built, and records price_xno, never price_usd, so USD limits and history totals are unchanged.
• wallet create/import/address/list/export/remove accept --chain nano; quotes lists nano:mainnet exact as supported with no USD price. README updated.

Tests
Published Nano key->address vectors (zero seed and the BIP-39 path), signature and block-hash checks, and end-to-end pay against a local 402 merchant and mock node: settle, dry-run, missing cap, over cap, node error -> network_error, bad payTo checksum, xrb_ payTo, mainnet-only, retry without double pay, and Base still selected with --chain base when both rails are offered.

Ran: bun install --frozen-lockfile && bun run lint && bun run typecheck && bun run knip && bun run test -- --coverage && bun run build

• Before: lint/typecheck/knip clean; 52 files passed, 1 skipped; 547 tests passed, 2 skipped; build ok.
• After: lint/typecheck/knip clean; 54 files passed, 1 skipped; 586 tests passed, 2 skipped; coverage 90.53/85.21/92.95/92.22; build ok.

Limits: Nano is not under the global USD limits (only the per-call --max-spend-xno); check, discover, balance, fund, send do not take nano yet; an unopened account reads as insufficient balance.