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"]

apps/darkroom-android/src/lib.rs

@@ -0,0 +1,36 @@
+//! DarkRoom Android entry point.
+//!
+//! The counterpart to `darkroom-desktop`'s `main`, with two differences that
+//! come from the platform rather than from choice:
+//!
+//! * There are no command-line paths. Android's SAF hands out document URIs,
+//!   not filesystem paths (ARCH §6.9), so the viewer opens with an empty
+//!   browsing list and the library grid is the only way in.
+//! * Logging goes to logcat. `env_logger` writes to stderr, which Android
+//!   discards.
+
+/// TRACES: M-13 | M-14
+/// Android application entry point, called by android-activity's glue.
+#[no_mangle]
+fn android_main(app: slint::android::AndroidApp) {
+    android_logger::init_once(
+        android_logger::Config::default()
+            .with_max_level(log::LevelFilter::Info)
+            .with_tag("DarkRoom"),
+    );
+
+    log::info!("DarkRoom v{}", env!("CARGO_PKG_VERSION"));
+
+    if let Err(e) = slint::android::init(app) {
+        log::error!("Slint Android backend failed to initialise: {e}");
+        return;
+    }
+
+    // Empty rather than the desktop's argv: see the module note above.
+    //
+    // Returning from `android_main` ends the process, so a failure here is
+    // logged rather than propagated — there is no shell to show `Err` to.
+    if let Err(e) = dr_ui::run(Vec::new()) {
+        log::error!("DarkRoom exited with error: {e:#}");
+    }
+}