ant-ffi: Client constructors should accept an explicit data_dir (Android needs it)

[Nic-dorman]

Problem

ant-core::config::home_dir() reads $HOME (or $USERPROFILE) unconditionally and returns Error::HomeDirNotFound if neither is set. This trips every ant-ffi Client constructor on Android, because Android app processes don't inherit a HOME env var by default — Zygote/ART set up the process environment minimally.

Concrete repro: in the AntAndroidDemo (PR adds it to ant-android/Examples/) or in the SmokeTest in AntAndroidSmoke, calling any Client.connectXxx() from a freshly launched Android app fails immediately with:

uniffi.ant_ffi.InternalException: called `Result::unwrap()` on an `Err` value: HomeDirNotFound

Current workaround

Plant HOME via libc setenv before the first FFI call (we use JNA in the demo/smoke):

internal interface LibC : Library {
    fun setenv(name: String, value: String, overwrite: Int): Int
    companion object { val INSTANCE: LibC = Native.load("c", LibC::class.java) }
}
// Once at app startup:
LibC.INSTANCE.setenv("HOME", context.filesDir.absolutePath, 1)

This works but it's ugly, racy across multi-process apps, and obscures the real intent (the app is choosing where the SDK should put its data).

Proposed fix

Add an explicit data_dir: String (or Option<String>) parameter to the FFI constructors:

• Client.connect_local(data_dir: Option<String>)
• Client.connect(peers, data_dir: Option<String>)
• Client.connect_with_wallet(..., data_dir: Option<String>)
• Client.connect_from_devnet_manifest(path, data_dir: Option<String>)

When None, fall back to the current ant_core::config::data_dir() behaviour (so existing macOS/iOS/Linux callers don't have to change anything).

On Android, consumers pass Context.filesDir.absolutePath. On iOS, the app's sandbox documents/library dir. On macOS/Linux, can stay None.

Related

Surfaced during the runtime smoke for ant-android v0.0.1 — see PR #156. The libc shim is committed in AntAndroidDemo and the smoke test until this lands.