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/build.rs

@@ -0,0 +1,13 @@
+// Copyright The Hush Developers  2019-2022
+// Released under the GPLv3
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    tonic_build::configure()
+         .build_server(false)
+         .compile(
+            &["proto/service.proto", "proto/compact_formats.proto"],
+            &["proto"],
+         )?;
+    println!("cargo:rerun-if-changed=proto/service.proto");
+    Ok(())
+ }
+