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/lib/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "qtlib"
+version = "0.1.0"
+authors = ["zecwallet", "The Hush Developers"]
+edition = "2018"
+
+[lib]
+name = "silentdragonxlite"
+crate-type = ["staticlib"]
+
+[dependencies]
+libc = "0.2.58"
+lazy_static = "1.4.0"
+blake3 = "0.3.4"
+silentdragonxlitelib = { path = "../silentdragonxlite-cli/lib" }
+socket2 = "0.3.11"