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

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.

No files, tests, or entry points are named. Start by locating the current source-build path and how the resolved package version selects native artifacts, then map the release download and checksum requirements. Done means prebuilt, source, and auto modes are defined, with version-locked verification and clear fallback instructions.

Written by the indexing model from the issue text.