Feat: add napi-rs Node.js bindings

[EnderRomantice]

Add the @holt/node package with Buffer/Uint8Array Tree APIs, bigint versions, scans, WAL reopen coverage, and napi-rs generated loader/types. Keep native artifacts out of git and document local/release builds.

What changed

Added a new packages/holt-node workspace member that provides Node.js bindings for Holt through napi-rs.

The package:

• Binds directly to the Rust holt crate.
• Exposes Tree.open, Tree.openMemory, get, getRecord, put, delete, compareAndPut, checkpoint, scanKeys, scanRecords, and close.
• Accepts Buffer and Uint8Array keys and values.
• Exposes record versions as JavaScript bigint.
• Supports prefix scans, startAfter, and delimiter common-prefix rollups.
• Generates the Node-API loader and TypeScript declarations.
• Keeps platform-specific .node artifacts out of Git.
• Documents local native builds and the future multi-platform release packaging flow.
• Keeps the existing holt-ffi C ABI unchanged.

Test plan

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo test --workspace --lib --tests --locked
• cargo test -p holt-node --locked
• npm test in packages/holt-node
• Built the native addon with npm run build
• Packed the package with npm pack and installed the tarball into a standalone Next.js application.
• Verified next build and next start.
• Verified server-side CRUD, prefix scan, checkpoint/reopen, and bigint version handling through Next.js.
• Fuzzing was not run; the change adds a binding layer without changing storage or WAL logic.
• Coverage was not rerun; no existing Rust storage paths were modified.
• Full rustdoc validation was not rerun.

No unsafe block or on-disk layout was added or changed.

Related

No related issue. This is the initial Node.js binding layer for Holt.