feat(ffi): Node.js bindings (napi-rs)

[Nic-dorman]

What

Adds ffi/node — a Node.js / Electron binding for the daemon-less Autonomi
client, the sibling of the Python effort. UniFFI has no maintained Node backend,
so this is a hand-written napi-rs wrapper: #[napi] types that
delegate to ant-ffi's plain lib target. No business logic lives in this crate —
it mirrors the same inner client the Swift/Kotlin/C#/Python bindings wrap.

Published as @withautonomi/ant-ffi (scoped, distinct from the daemon SDK's
antd package). Install pulls a prebuilt per-platform .node — no Rust toolchain,
no node-gyp.

Surface

• 9 connect* constructors (local / default / default+wallet /
  default-external-signer / custom peers ± wallet / devnet-manifest ±
  external-signer / external-signer), all with optional Android dataDir
• ~30 Client methods: chunks, public/private data, files (+progress),
  cost estimate (+progress), streaming downloads-to-file (+progress), data-map
  helpers, walletApprove, and the full external-signer flow
  (prepare* → paymentTransactions → finalizeUpload / finalizeUploadMerkle
  / cancelUpload)
• Wallet (fromPrivateKey/address/balanceOf*) + free fns
  antFfiVersion, networkInfo, waitForReceipt, merkleWinnerPoolHash
• 6 enums, ~15 records, auto-generated index.d.ts (TS autocomplete out of the box)

How it works

• async fn → Promise<T> on napi's tokio runtime; the event loop never blocks
• _with_progress methods bridge a plain JS (progress) => void callback to
  ant-ffi's ProgressListener via a napi ThreadsafeFunction (safe from the
  core's background progress task)
• Buffer/Uint8Array zero-copy for payloads; token/gas amounts stay decimal
  strings (they exceed 2^53)

Also re-exports ant-ffi's free functions at its crate root so this crate can call
them (previously only wired into the UniFFI scaffolding).

Testing

• ✅ Clean build, zero warnings; index.d.ts generates correctly
• ✅ 7/7 offline smoke tests (node:test): import, version, offline EVM-address
  derivation, networkInfo, full-surface presence, free-fn rejection
• ✅ Live devnet round-trip: paid public-data upload → byte-identical download;
  file upload with 10 progress callbacks across Encrypting→Quoting→Storing;
  file download byte-identical. Validates the async + progress-bridge + payment
  chain against ant-core 0.5.1 with zero protocol skew.

Not in this PR (follow-ups)

• Distribution/CI: the 8-target napi cross-compile GitHub Actions matrix +
  per-platform npm packages + napi prepublish (publish needs an npm org/token).
• Richer error mapping (machine-readable code per variant; PartialUpload's
  money-visible fields on the JS error).

Draft pending the CI/distribution follow-up and review.

🤖 Generated with Claude Code