ci: add npm publish workflow with cross-platform binary builds

[LeeroyHannigan]

Add npm publish workflow for cross-platform binary distribution

Summary

Enables users to install and run ExtendDB via npm without needing Rust or building from source:

npx extenddb init
npx extenddb serve

How it works

Uses the same pattern as esbuild, Prisma, and Turbo:

1. Platform packages (@extenddb/darwin-arm64, @extenddb/darwin-x64, @extenddb/linux-x64, @extenddb/linux-arm64) each contain a pre-built binary
2. Main package (extenddb) is a thin wrapper that detects the platform and delegates to the correct binary
3. npm's optionalDependencies ensures only the relevant platform package is downloaded

CI workflow

Triggered by a GitHub Release (from release-please):

1. Cross-compiles for 4 targets in parallel using the GitHub Actions matrix
2. Publishes platform packages first, then the main wrapper
3. Version is derived from the release tag (v0.1.0 → 0.1.0)

Files added

.github/workflows/npm-publish.yml — CI workflow
npm/extenddb/ — Main wrapper package
npm/extenddb/bin/extenddb — Platform-detecting CLI launcher
npm/@extenddb/darwin-arm64/ — macOS Apple Silicon
npm/@extenddb/darwin-x64/ — macOS Intel
npm/@extenddb/linux-x64/ — Linux x64
npm/@extenddb/linux-arm64/ — Linux ARM64

Prerequisites

• npm org @ExtendDB created on npmjs.com
• NPM_TOKEN secret added to the repo (Automation token from npmjs.com)
• release-please workflow merged (creates the GitHub Releases that trigger this)

Tested locally

Verified the wrapper correctly resolves and executes the platform binary:

$ node node_modules/.bin/extenddb version
extenddb 0.0.2
catalog 0.0.2 (postgres)