feat(desktop): add the core crate and a fixture-driven app window (HACK-2214)

.github/workflows/desktop.yml

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+# Hack desktop app (HACK-2214): unsupported preview.
+# Separate from the default gates and the release workflow, and not a
+# required check. Linux and macOS builds of the app plus the `core` tests.
+# The toolchain comes from apps/desktop/rust-toolchain.toml (1.98.1; macOS
+# needs it), not from the repo root pin.
+name: Desktop app
+
+on:
+  pull_request:
+    paths:
+      - "apps/desktop/**"
+      - ".github/workflows/desktop.yml"
+  push:
+    branches: [next]
+    paths:
+      - "apps/desktop/**"
+      - ".github/workflows/desktop.yml"
+
+permissions:
+  contents: read
+
+concurrency:
+  group: desktop-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  linux:
+    name: Desktop app (Linux build, core tests)
+    runs-on: ubuntu-24.04
+    timeout-minutes: 40
+    defaults:
+      run:
+        working-directory: apps/desktop
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install GPUI build dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y --no-install-recommends \
+            clang libfontconfig-dev libwayland-dev libxkbcommon-x11-dev \
+            libx11-xcb-dev libssl-dev libzstd-dev libvulkan-dev pkg-config
+      - uses: dtolnay/rust-toolchain@1.98.1
+        with:
+          components: rustfmt, clippy
+      - uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            apps/desktop/target
+          key: desktop-${{ runner.os }}-${{ hashFiles('apps/desktop/Cargo.lock') }}
+          restore-keys: desktop-${{ runner.os }}-
+      - name: Format
+        run: cargo fmt --all --check
+      - name: Core tests (no daemon, no Docker)
+        env:
+          HACK_DAEMON_AUTOSTART_DISABLE: "1"
+        run: cargo test --locked -p hack-desktop-core
+      # Both: the screenshots feature turns on gpui test-support, which can
+      # hide test-only APIs from a build that only uses that feature.
+      - name: Clippy (default features)
+        run: cargo clippy --locked --all-targets -- -D warnings
+      - name: Clippy (screenshots feature)
+        run: cargo clippy --locked --all-targets --features hack-desktop/screenshots -- -D warnings
+      - name: Build app
+        run: cargo build --locked -p hack-desktop
+
+  macos:
+    name: Desktop app (macOS build, core tests)
+    runs-on: macos-15
+    timeout-minutes: 40
+    defaults:
+      run:
+        working-directory: apps/desktop
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@1.98.1
+        with:
+          components: clippy
+      - uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            apps/desktop/target
+          key: desktop-${{ runner.os }}-${{ hashFiles('apps/desktop/Cargo.lock') }}
+          restore-keys: desktop-${{ runner.os }}-
+      - name: Core tests (no daemon, no Docker)
+        env:
+          HACK_DAEMON_AUTOSTART_DISABLE: "1"
+        run: cargo test --locked -p hack-desktop-core
+      - name: Clippy (default features)
+        run: cargo clippy --locked --all-targets -- -D warnings
+      - name: Build app
+        run: cargo build --locked -p hack-desktop

apps/desktop/.gitignore

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+/target/
+/screenshots/out/