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.

core/dr-gpu/src/error.rs

@@ -0,0 +1,23 @@
+/// Failures from the GPU layer.
+///
+/// `DeviceLost` is deliberately a distinct variant rather than folded into a
+/// generic error: it is an expected event on Android (ARCH §6.10), not an
+/// exceptional one, and callers recover from it by rebuilding the device and
+/// re-driving from the edit graph.
+#[derive(Debug, thiserror::Error)]
+pub enum GpuError {
+    #[error("no suitable GPU adapter found")]
+    NoAdapter,
+
+    #[error("failed to request device: {0}")]
+    DeviceRequest(String),
+
+    #[error("GPU device lost — recreate and re-render from the edit graph")]
+    DeviceLost,
+
+    #[error("shader compilation failed: {0}")]
+    ShaderCompilation(String),
+
+    #[error("readback failed: {0}")]
+    Readback(String),
+}