feat: add client for handling ldk-node backups

[ovitrif]

Context

The app's VSS client was using a truncated 32-byte seed for all key derivation, while ldk-node uses the full 64-byte BIP39 seed. This made it impossible to read ldk-node's VSS data from the app layer. Additionally, ldk-node uses a namespaced key format (obf("primary#secondary")#obf("key")) that the flat-key APIs couldn't address.

Changes

Dual key derivation

The client derives two independent key sets during initialization:

• App keys (store/get/delete/list_keys/list): derived from the truncated 32-byte seed, preserving backward compatibility with existing app backups
• LDK keys (via dedicated LdkVssClient): derived from the full 64-byte seed, matching ldk-node's internal key derivation

LNURL auth is derived from the app seed path (truncated 32-byte), maintaining the same server identity as v0.4.0.

Dedicated LDK client with LdkNamespace enum

Decoupled LDK operations into a standalone LdkVssClient (ldk_client.rs), completely separate from the app backup VssClient. Each has its own global singleton, initialization, and shutdown:

• vss_new_ldk_client_with_lnurl_auth() / vss_shutdown_ldk_client()
• vss_ldk_get(), vss_ldk_store(), vss_ldk_delete(), vss_ldk_list_keys(), vss_ldk_list_all_keys()

All accept a type-safe LdkNamespace enum (defined in Rust, generated for Kotlin/Swift by UniFFI):

pub enum LdkNamespace {
    Default,                                // ("", "")
    Monitors,                               // ("monitors", "")
    MonitorUpdates { monitor_id: String },   // ("monitor_updates", monitor_id)
    ArchivedMonitors,                        // ("archived_monitors", "")
}

This ensures the LDK client uses only ldk-node's key derivation chain (full 64-byte seed → m/877' → HKDF) with no coupling to the app client's truncated-seed derivation.

CI

Aligns the Gradle publish workflow with bitkit-core's pattern.

QA Notes

• cargo test passes (19/19)
• All bindings regenerated (Android, iOS, Python)
• Release: v0.5.12
• Android package: com.synonym:vss-client-android:0.5.12
• Tested in: fix: delete vss network graph using new ffi client bitkit-android#775

[ovitrif]

Drafted to fix keyspace for ldk APIs

[pwltr]

Thanks for doing the release as well, will test now on iOS end