Initial workspace: GPU context, compute pass, adaptive Slint shell

Establishes the v0.1 foundations on both platforms:

- dr-types: SourceRef (never a filesystem path — Android SAF has none),
  Format, Availability, Validator with ETag quote normalisation
- dr-gpu: wgpu device, compute pass writing a storage texture, resize
- dr-ui: Slint shell with FR-UI-1 adaptive layout, computed in Rust to
  avoid a binding loop
- docker/android: pinned toolchain, verified producing API 28 ARM binaries

Measured the cost of the temporary CPU readback path (dr-gpu bench):
compute is 0.06-0.28ms across sizes while readback is 0.63-7.43ms, so
readback is 90-96% of frame time and scales with area. Recorded in
ARCH §6.1 — this is why spike S1 is the priority.

Mitigations pending S1: reuse the staging buffer, apply at most one
resize per frame, and cap render resolution at 2048 on the long edge.

10 tests passing; core crates cross-compile for aarch64-linux-android.

apps/darkroom-desktop/src/main.rs

@@ -0,0 +1,11 @@
+//! DarkRoom desktop entry point.
+
+fn main() -> anyhow::Result<()> {
+    env_logger::Builder::from_env(
+        env_logger::Env::default().default_filter_or("info,wgpu_core=warn,wgpu_hal=warn,zbus=warn,tracing=warn,calloop=warn"),
+    )
+    .init();
+
+    log::info!("DarkRoom v{}", env!("CARGO_PKG_VERSION"));
+    dr_ui::run()
+}