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

apps/darkroom-android/Cargo.toml

@@ -0,0 +1,28 @@
+[package]
+name = "darkroom-android"
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+
+# A cdylib, not a bin: Android loads the app as a shared library and calls
+# `android_main` through android-activity's glue. Nothing execs a binary, so
+# there is no `main` to provide.
+[lib]
+name = "darkroom"
+crate-type = ["cdylib"]
+
+[dependencies]
+# No backend feature to select: dr-ui picks its Slint backend from the target,
+# so building for aarch64-linux-android gets android-activity automatically.
+dr-ui.workspace = true
+anyhow.workspace = true
+log.workspace = true
+android_logger = "0.15"
+
+[features]
+# Mirrors darkroom-desktop: the CPU readback path stays on until spike S1
+# lands zero-copy. On Adreno this is the same wrong path as on desktop, only
+# with less memory bandwidth to absorb it (ARCH §6.1).
+default = ["readback"]
+readback = ["dr-ui/readback"]