feat(lite): vendor SDXL backend source; source deps from git.dragonx.is

Vendor the two local Rust crates that build the lite backend artifact into
third_party/silentdragonxlite/ (the qtlib C-ABI wrapper + the silentdragonxlitelib
core, with proto/res and all the lite-send fixes), and point
build-lite-backend-artifact.sh's default --backend-dir there, so the lite wallet
builds without the upstream SilentDragonXLite repo.

External build inputs are now only the Rust toolchain + git.dragonx.is + crates.io:
- the 6 librustzcash git deps point at the git.dragonx.is/DragonX/librustzcash
  mirror (pinned rev acff1444), not git.hush.is;
- the Sapling params are gitignored (not committed, no Git LFS) - the build fetches
  them from the git.dragonx.is/DragonX/zcash-params 'sapling-v1' release and verifies
  their SHA-256 before rust-embed bakes them in (ensure_sapling_params).

For fully offline builds, cargo vendor into lib/vendor/ and add a vendored-sources
redirect (vendor/ is gitignored; the script symlinks it into the prepared dir).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

third_party/silentdragonxlite/silentdragonxlite-cli/lib/src/lib.rs

@@ -0,0 +1,29 @@
+// Copyright The Hush Developers  2019-2022
+// Released under the GPLv3
+#[macro_use]
+extern crate rust_embed;
+
+pub mod lightclient;
+pub mod grpcconnector;
+pub mod lightwallet;
+pub mod commands;
+
+#[cfg(feature = "embed_params")]
+#[derive(RustEmbed)]
+#[folder = "zcash-params/"]
+pub struct SaplingParams;
+
+#[derive(RustEmbed)]
+#[folder = "res/"]
+pub struct PubCertificate;
+
+
+// Anchor depth back from the tip for shielded spends. MUST be > 0: with 0 the wallet anchors to
+// the absolute chain tip, which races the node committing that anchor and intermittently triggers
+// "bad-txns-shielded-requirements-not-met" (missing sapling anchor) on broadcast. 4 matches upstream
+// zecwallet-lite and uses a confirmed, reorg-stable anchor.
+pub const ANCHOR_OFFSET: u32 = 4;
+
+pub mod grpc_client {
+    tonic::include_proto!("cash.z.wallet.sdk.rpc");
+}