chore: add Nix devshell for aarch64-linux-android cross-compilation

[agentotto]

Summary

Adds a minimal Nix flake for WalletKit development and Android libwalletkit.so builds without Docker or cross-rs.

The flake provides:

• default dev shell: pinned Rust toolchain, curl, and git for host development
• android dev shell: full Android cross-compilation environment for aarch64-linux-android
• Shells for x86_64-linux, aarch64-darwin, and x86_64-darwin
• Rust from rust-toolchain.toml via oxalica/rust-overlay
• Android NDK r27 (27.2.12479018) via androidenv.composeAndroidPackages
• Platform-specific NDK toolchain paths:
  • Linux: toolchains/llvm/prebuilt/linux-x86_64/bin
  • macOS: toolchains/llvm/prebuilt/darwin-x86_64/bin
• curl for walletkit-db/build.rs SQLite3MC downloads
• Android cross-compiler/linker env vars for aarch64-linux-android
• 16KB page-size Rust linker flags matching .cargo/config.toml

No existing CI workflows or Cross.toml are modified.

Usage

For a pinned Rust host-development environment:

nix develop

For Android cross-compilation:

nix develop .#android

Then build the mobile release artifact:

cargo build -p walletkit --release --target aarch64-linux-android --features compress-zkeys,v3

Or use the helper:

./nix/build-android.sh

Verification

In the workspace I verified:

nix flake check --all-systems --no-build --no-write-lock-file

The shell evaluates for all configured systems.

I could not complete a real nix develop .#android --command cargo build ... in this workspace because the provided pod is aarch64-linux, while this flake intentionally provides the requested x86_64-linux and Darwin shells. Attempting to realize the x86_64 shell on the aarch64 pod fails with Exec format error while running x86_64 Nix builders. This should be validated on an amd64 Linux runner or a supported macOS machine.

Next steps

• Validate the full Android build on an amd64 Linux or macOS machine/runner.
• Add additional Android targets once the aarch64 flow is proven.
• Wire this shell into CI as a follow-up, replacing the Android cross-rs path incrementally.