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/Makefile

@@ -0,0 +1,28 @@
+ifeq ($(shell uname),Darwin)
+    EXT := dylib
+	CFLAGS := "-mmacosx-version-min=10.11"
+else
+    EXT := a
+	CFLAGS :=
+endif
+
+PWD := $(shell pwd)
+
+all: release
+
+winrelease: target/x86_64-pc-windows-gnu/release/silentdragonxlite.lib
+
+target/x86_64-pc-windows-gnu/release/silentdragonxlite.lib: src/lib.rs Cargo.toml
+	SODIUM_LIB_DIR="$(PWD)/libsodium-mingw/" cargo build --lib --release --target x86_64-pc-windows-gnu
+
+release: target/release/silentdragonxlite.$(EXT)
+debug: target/debug/silentdragonxlite.$(EXT) 
+
+target/release/silentdragonxlite.$(EXT): src/lib.rs Cargo.toml
+	LIBS="" CFLAGS=$(CFLAGS) cargo build --lib --release
+
+target/debug/silentdragonxlite.$(EXT): src/lib.rs Cargo.toml
+	LIBS="" CFLAGS=$(CFLAGS) cargo build --lib
+
+clean:
+	rm -rf target