Support prebuilt binaries as default (with opt-in source builds)

[BitcoinZavior]

Right now building from source is the default method, which means every user of this repository will need Rust + NDK + platform toolchains set up. This adds a lot of friction for regular Flutter devs who just want to add bdk as a dependency.

Proposal: Support multiple build modes:

Prebuilt - download from GitHub releases, verify checksums, no toolchain needed
Source - opt-in for custom builds, auditing, airgapped environments etc
Auto (default) - try prebuilt first, fall back to source if unavailable or verification fails

Binaries should be version-locked to the resolved package version and verified with SHA256. If download fails or checksum doesn't match, show a clear error with instructions to switch to source mode.
This keeps the zero-setup experience for most users while still supporting advanced use cases.

[reez]

Thanks for opening this. +1

#12 moved us to native assets (the recommended Dart/Flutter path), so the default experience now requires Rust + platform toolchains, which may or may not be preferred across every single Flutter dev, so I agree it’s worth potentially addressing with a precompiled option (auto/always/never) that still allows source builds when needed.

[BitcoinZavior]

Yes, please take a look as it needs a bit of setup too, like secrets etc. This PR will also provide signed binaries.