Feat: Prebuilt Binaries

[Psycarlo]

Prebuilt native binaries + Expo support

Makes bdk-rn installable with a plain npm install / npx expo install. No Rust toolchain, no manual tarball download.

How it works

• Release workflow (.github/workflows/release.yml): pushing a v* tag builds the Rust binaries (Android: arm64-v8a, armeabi-v7a, x86, x86_64 · iOS: device + arm64/x86_64 simulators), attaches them as zips to a GitHub Release with SHA-256 checksums and build provenance attestation, then publishes to npm via trusted publishing. Checksums are injected into the published package.json; binaries are stripped so the tarball stays small.
• Postinstall (scripts/postinstall.js): downloads the zips for the matching release, verifies checksums, extracts them into place. Supports BDK_RN_SKIP_POSTINSTALL, BDK_RN_BINARY_BASE_URL (mirror), and BDK_RN_ARTIFACTS_DIR (offline installs).
• Expo config plugin (app.plugin.js): fails prebuild with a clear error if the New Architecture is disabled; sets required gradle properties.

Before the first release

• npm access to the bdk-rn package name
• Configure this repo as a trusted publisher on npmjs.com
• Bump the version (the v1.0.0 tag already exists; the workflow refuses existing releases)