Add the Android app shell; cap cross-build parallelism

Cap both halves of the container build: CARGO_BUILD_JOBS limits how many
rustc processes cargo starts, while --cpus limits what the container gets
regardless of what nested build scripts spawn — cc, cmake, and ring's asm
build all parallelise on their own account and do not consult cargo. Without
both, a full cross-compile takes every thread on the host and makes the
machine unusable for the length of a background build.

Assisted-by: LLM

Cargo.toml

@@ -13,6 +13,7 @@ members = [
     "platform/dr-plat",
     "ui/dr-ui",
     "apps/darkroom-desktop",
+    "apps/darkroom-android",
     "tools/traceability",
 ]