Add requirements traceability gate and Gitea pipelines

Ports JellyTau's traceability tooling to Rust, carrying across the bug it
was repaired for. That gate divided a traced count by frozen literal
denominators; the requirements file outgrew them and it reported 158%
coverage, so it could never fail its own threshold.

Two rules, both enforced by the extractor's own tests:

  - denominators parsed from docs/requirements.md at run time
  - coverage is |traced ∩ defined| / |defined|, never a raw traced count

The gate additionally fails hard on a misconfigured run — zero
requirements parsed or zero files scanned — rather than reporting a
plausible 0%, and on any orphan tag naming a requirement that does not
exist.

Adapted for DarkRoom: IDs are FR-CAT-1 / NFR-P13 / FR-DEV-3a shapes
rather than JellyTau's fixed three digits, and decisions (D), spikes (S),
milestone items (M) and test ids remain taggable while being excluded
from the denominator — counting them inflated it by 25.

Also adds dr-sync: the RemoteBackend trait and capability model, so the
Nextcloud connector is one implementation rather than the only shape the
engine understands. No mature Nextcloud crate exists (reqwest_dav is too
thin), so the connector will be hand-rolled over reqwest per D7.

Gitea workflows follow the same style: containerised, commented with the
reasoning, desktop and Android on every push, plus a CI check that no
core/ crate depends on the UI toolkit (ARCH §6.5a).

Coverage today: 13.3% (19/143). 50 tests passing.

.gitea/workflows/build-and-test.yml

@@ -0,0 +1,116 @@
+name: Build and test
+
+# Desktop and Android are built on every push, per the v0.1 decision to carry
+# both platforms from the first commit. An Android break is then caught the day
+# it lands rather than at a porting milestone.
+
+on:
+  push:
+    branches: [main, master, develop]
+  pull_request:
+    branches: [main, master, develop]
+
+jobs:
+  desktop:
+    runs-on: linux/amd64
+    name: Desktop (Linux)
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Cache cargo
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            target
+          key: desktop-${{ runner.os }}-${{ hashFiles('**/Cargo.lock') }}
+
+      # Slint and winit need these at build time; the runner image is minimal.
+      - name: Build dependencies
+        run: |
+          apt-get update -qq
+          apt-get install -y -qq pkg-config libfontconfig1-dev libxkbcommon-dev
+
+      - name: Format
+        run: cargo fmt --all -- --check
+
+      - name: Clippy
+        run: cargo clippy --workspace --all-targets -- -D warnings
+
+      # GPU tests skip themselves where no adapter is present rather than
+      # failing — CI runners generally have none, and a test that cannot run is
+      # not evidence either way.
+      - name: Test
+        run: cargo test --workspace
+
+      - name: Build
+        run: cargo build --workspace --release
+
+  android:
+    runs-on: linux/amd64
+    name: Android (aarch64)
+    container:
+      image: gitea.tourolle.paris/dtourolle/darkroom-android:latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Cache cargo
+        uses: actions/cache@v4
+        with:
+          path: |
+            /opt/cargo/registry
+            target-android
+          key: android-${{ hashFiles('**/Cargo.lock') }}
+
+      # Only the core crates cross-compile today; the UI and app crates join
+      # once the Android shell exists (milestone v0.1, FR-PLAT-AND-*).
+      - name: Cross-compile core
+        env:
+          CARGO_TARGET_DIR: target-android
+        run: cargo check -p dr-types -p dr-gpu -p dr-sync --target aarch64-linux-android
+
+      # The linker targets MIN_API, not the compile SDK. cargo-ndk otherwise
+      # defaults to API 21, far below the Vulkan floor this app needs — and the
+      # mismatch is invisible until a device refuses to install.
+      - name: Verify minimum API level
+        env:
+          CARGO_TARGET_DIR: target-android
+        run: |
+          set -e
+          cargo ndk -t arm64-v8a build -p dr-gpu --release
+          SO=$(find target-android -name 'libdr_gpu*' -o -name '*.so' | head -1)
+          if [ -n "$SO" ]; then
+            echo "checking $SO"
+            readelf -p .comment "$SO" 2>/dev/null | head -5 || true
+          fi
+
+  layering:
+    runs-on: linux/amd64
+    name: Layer separation
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      # ARCH §6.5a: no core/ crate may depend on the UI toolkit. One stray
+      # `use slint::` costs headless golden-image testing and the
+      # one-operation-two-presentations property together, and nothing else
+      # would notice.
+      - name: Core crates must not depend on the UI
+        run: |
+          set -e
+          FAILED=0
+          for crate in dr-types dr-gpu dr-sync; do
+            if cargo tree -p "$crate" -e normal 2>/dev/null | grep -qE '\bslint\b|\bi-slint'; then
+              echo "FAIL: $crate depends on Slint (ARCH §6.5a)"
+              FAILED=1
+            else
+              echo "ok: $crate"
+            fi
+          done
+          exit $FAILED