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.

Cargo.toml

@@ -0,0 +1,46 @@
+[workspace]
+resolver = "2"
+members = [
+    "core/dr-types",
+    "core/dr-gpu",
+    "ui/dr-ui",
+    "apps/darkroom-desktop",
+]
+
+[workspace.package]
+version = "0.1.0"
+edition = "2021"
+rust-version = "1.92"
+license = "GPL-3.0-or-later"
+repository = "https://github.com/dtourolle/DarkRoom"
+
+[workspace.dependencies]
+# Internal
+dr-types = { path = "core/dr-types" }
+dr-gpu = { path = "core/dr-gpu" }
+dr-ui = { path = "ui/dr-ui" }
+
+# GPU + UI
+wgpu = "23"
+slint = { version = "1.9", default-features = false }
+slint-build = "1.9"
+
+# Foundations
+anyhow = "1"
+thiserror = "2"
+log = "0.4"
+env_logger = "0.11"
+pollster = "0.4"
+bytemuck = { version = "1", features = ["derive"] }
+
+[profile.dev]
+# Dependencies optimised even in dev builds — wgpu and image decoding are
+# unusably slow otherwise, and they rarely need debugging.
+opt-level = 0
+
+[profile.dev.package."*"]
+opt-level = 2
+
+[profile.release]
+lto = "thin"
+codegen-units = 1