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.

docker/android/build.sh

@@ -0,0 +1,59 @@
+#!/usr/bin/env bash
+# Run a command inside the DarkRoom Android build container.
+#
+#   ./docker/android/build.sh cargo ndk -t arm64-v8a build --release
+#   ./docker/android/build.sh cargo check --target aarch64-linux-android
+#   ./docker/android/build.sh                    # interactive shell
+#
+# Builds the image on first use. Rebuild after editing the Dockerfile with:
+#   ./docker/android/build.sh --rebuild
+set -euo pipefail
+
+IMAGE="darkroom-android:latest"
+HERE="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+REPO="$(cd "${HERE}/../.." && pwd)"
+
+# Prefer podman (rootless by default); fall back to docker.
+if command -v podman >/dev/null 2>&1; then
+    ENGINE=podman
+elif command -v docker >/dev/null 2>&1; then
+    ENGINE=docker
+else
+    echo "error: neither podman nor docker found" >&2
+    exit 1
+fi
+
+if [[ "${1:-}" == "--rebuild" ]]; then
+    shift
+    "${ENGINE}" build -t "${IMAGE}" "${HERE}"
+elif ! "${ENGINE}" image exists "${IMAGE}" 2>/dev/null && \
+     ! "${ENGINE}" image inspect "${IMAGE}" >/dev/null 2>&1; then
+    echo "==> building ${IMAGE} (first run; several minutes)"
+    "${ENGINE}" build -t "${IMAGE}" "${HERE}"
+fi
+
+# Persist the cargo registry and target dir across runs, or every build
+# re-downloads the crate index.
+CACHE="${XDG_CACHE_HOME:-${HOME}/.cache}/darkroom-android"
+mkdir -p "${CACHE}/registry" "${CACHE}/target"
+
+ARGS=(
+    --rm
+    -v "${REPO}:/work:z"
+    -v "${CACHE}/registry:/opt/cargo/registry:z"
+    -v "${CACHE}/target:/work/target-android:z"
+    -e CARGO_TARGET_DIR=/work/target-android
+    -w /work
+)
+
+# Rootless podman already maps the host user; docker needs it stated.
+if [[ "${ENGINE}" == "docker" ]]; then
+    ARGS+=(--user "$(id -u):$(id -g)")
+fi
+
+if [[ $# -eq 0 ]]; then
+    ARGS+=(-it)
+    set -- /bin/bash
+fi
+
+exec "${ENGINE}" run "${ARGS[@]}" "${IMAGE}" "$@"