Add reproducible binary verification workflow

[KingParmenides]

Companion to SatoshiPortal/bdk-flutter#5 and the reproducible-build bounty in SatoshiPortal/bdk-flutter#1.

This ports the same Cargokit reproducibility verifier to boltz-dart, one of the sibling libraries named in the bounty.

What changed:

• Added build_tool reproduce-binaries, which rebuilds selected Rust targets locally, downloads the matching precompiled_<crate-hash> GitHub release assets, verifies Ed25519 signatures, and byte-compares local vs remote artifacts.
• Added --url-prefix and --public-key overrides for release verification before committing a precompiled_binaries section to rust/cargokit.yaml.
• Added bash/Docker wrappers for Linux and Android reproducibility checks.
• Added a macOS wrapper for macOS/iOS targets.
• Documented direct and wrapper usage in cargokit/docs/precompiled_binaries.md and reproducible_builds/README.md.

Verification performed:

• dart format lib/src/build_tool.dart lib/src/reproduce_binaries.dart
• dart pub get with Dart 3.4.4
• dart test with Dart 3.4.4
• dart analyze with Dart 3.4.4 (only existing info: unused private constructor in options.dart)
• dart run build_tool reproduce-binaries --help
• dart run build_tool reproduce-binaries --manifest-dir=../../rust --target not-a-target
• bash -n reproducible_builds/*.sh
• git diff --check

Payout BTC address for the linked bounty: 39Q34P8A7g375yqEr8buvNJkUbRgKfbKQZ