chore: prepare 0.5.18 artifacts

[pwltr]

Prepares the v0.5.18 package metadata after the shared SeedQR decoder landed in #155.

Description

• Bumps the Rust crate and lockfile from 0.5.17 to 0.5.18.
• Bumps the Android package version to 0.5.18.
• Points SwiftPM at the future v0.5.18 release asset.
• Records the checksum for the exact locally generated BitkitCore.xcframework.zip: c2bae82e20dd485cc82285cff7667d487d0c64bbd6f4686cb4e8f599e2f6a1eb.
• Leaves generated binaries untracked; the iOS ZIP will be attached to the GitHub Release and Android artifacts will be regenerated by the release workflow.

Preview

N/A — no UI changes.

QA Notes

Local release validation completed successfully:

• ./build.sh -r --patch all
• Android Maven-local publication completed for all four configured ABIs.
• cargo test modules::seedqr — 9 passed.
• cargo metadata --locked --no-deps --format-version 1
• swift package dump-package
• bash -n build.sh build_ios.sh build_android.sh build_python.sh
• swift package compute-checksum dist/ios/BitkitCore.xcframework.zip matched the checksum in Package.swift.

After merge, tag the merged commit as v0.5.18, attach the exact XCFramework ZIP, and publish the GitHub Release to trigger Android package publication.