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.

README.md

@@ -0,0 +1,48 @@
+# DarkRoom
+
+A cross-platform, non-destructive RAW photo editor for Linux and Android.
+
+**Status:** early. v0.1 is a remote library viewer — see
+[docs/milestone-v0.1.md](docs/milestone-v0.1.md).
+
+## Documentation
+
+| Document | Contents |
+|---|---|
+| [requirements.md](docs/requirements.md) | What the software must do — 122 numbered requirements |
+| [architecture.md](docs/architecture.md) | How it is built — crates, GPU pipeline, data model, sync |
+| [milestone-v0.1.md](docs/milestone-v0.1.md) | The first buildable milestone |
+
+## Building
+
+Desktop:
+
+```bash
+cargo run -p darkroom-desktop
+```
+
+Android (containerised toolchain, see [docker/android](docker/android/README.md)):
+
+```bash
+./docker/android/build.sh cargo ndk -t arm64-v8a build --release
+```
+
+## Current state
+
+Working: workspace, GPU context and compute pass, adaptive Slint shell, Android
+cross-compilation of the core crates.
+
+**Not yet working:** the zero-copy display path. The build currently uploads
+frames through the CPU, which is exactly what
+[ARCH §6.1](docs/architecture.md) forbids — measured at 96% of frame time at
+4K. Replacing it is spike S1, the project's highest priority.
+
+```
+cargo run -p dr-gpu --example bench --features readback
+```
+
+reproduces that measurement.
+
+## Licence
+
+GPL-3.0-or-later.