feat(payments): reuse cached credentials in send [AMB-3309]

[bufo24]

Problem

Transactions.send() derives node credentials on every call when a password is passed: two GraphQL round trips plus two Argon2id passes (m=64 MiB, t=3, p=4). Only prepareSend() filled the cache, and only a send() without a password read it.

Measurement (litd-mainnet-144d77c6, 100 sat payments to one Lightning address, one at a time)

Time from the send() call to the REST request reaching the node:

Mode	Runs	Range	Median
send() with password (derives every time)	10	8.4 s to 47.6 s (8.4 s to 14.6 s without one outlier)	10.3 s
prepareSend() once, then send() without password	5	1.2 s to 3.3 s	1.4 s

Change

• send() and prepareSend() now share one private path (#prepare). A cache hit is used even when a password is passed. A miss derives once and stores the result, so the first payment warms the cache.
• Concurrent sends for one wallet share the existing in-flight derivation (#pending).
• Kept: a failed derivation is never cached; forgetSend() during a derivation stops the result from being stored (that one send() still pays with what it derived); sandbox wallets work; retryPayment reads the same cache.
• Docs updated: README, docs/INTEGRATION.md, AGENTS.md, JSDoc on send, prepareSend, isSendReady, forgetSend, PaymentsConfig.send and SendParams.password.
• No new options or APIs.

Behavior changes to review

1. A decrypted node admin macaroon now stays in process memory after the first send(), with no expiry. Before, this happened only after an explicit prepareSend().
2. A wrong password no longer fails a send() when credentials are already cached. It still must be non-empty.
3. If node credentials rotate, callers must call forgetSend(walletId).

Also: a teamId override passed to send() is ignored on a cache hit.

Open question

Do we want an opt-out (for example a per-call flag to skip the cache) for callers that must not keep the macaroon in memory? Not added here; forgetSend() after a run of sends is the current way.

Test plan

• pnpm build passes
• pnpm typecheck passes (the repo has no lint script)
• pnpm format:check passes
• pnpm test: core 11/11, payments 82/82 pass
• New tests were written first and failed on the old code. They cover: cache hit with a different password (no derivation calls), first send warms the cache, consecutive sends derive once, concurrent sends share one derivation, send plus in-flight prepareSend share one, failed derivation not cached, forgetSend mid-derivation not stored, second sandbox send skips the wallet lookup.
• No live payments were run.

Linear: https://linear.app/amboss/issue/AMB-3309

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