ci: add nix flake verification and build job

.github/workflows/nix.yml

@@ -0,0 +1,66 @@
+name: nix
+
+on:
+  push:
+    branches:
+      - main
+    paths:
+      - flake.nix
+      - flake.lock
+      - .nix/**
+      - Cargo.toml
+      - Cargo.lock
+      - .github/workflows/nix.yml
+  pull_request:
+    branches:
+      - main
+    paths:
+      - flake.nix
+      - flake.lock
+      - .nix/**
+      - Cargo.toml
+      - Cargo.lock
+      - .github/workflows/nix.yml
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: nix-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  nix:
+    runs-on: ubuntu-latest
+    timeout-minutes: 60
+    steps:
+      - uses: actions/checkout@v6
+
+      - uses: cachix/install-nix-action@v31
+
+      # skia-bindings' build script does NOT validate the archive passed via
+      # SKIA_BINARIES_URL (upstream `// TODO: verify key`), so a stale pin
+      # builds successfully against outdated skia binaries instead of failing.
+      # Enforce the version match explicitly.
+      - name: Check skia-binaries pins match Cargo.lock
+        run: |
+          crate_version=$(grep -A1 'name = "skia-bindings"' Cargo.lock | sed -n 's/^version = "\(.*\)"/\1/p')
+          test -n "$crate_version" || { echo "::error::skia-bindings not found in Cargo.lock"; exit 1; }
+          grep -oE 'https://github.com/rust-skia/skia-binaries/releases/download/[^"]+' flake.nix | while read -r url; do
+            tag=$(echo "$url" | cut -d/ -f8)
+            if [ "$tag" != "$crate_version" ]; then
+              echo "::error::flake.nix pins skia-binaries $tag but Cargo.lock has skia-bindings $crate_version: $url"
+              exit 1
+            fi
+          done
+          echo "All skia-binaries pins match skia-bindings $crate_version"
+
+      - name: Verify all pinned flake inputs (all platforms)
+        run: nix flake archive
+
+      - name: Evaluate flake outputs for all systems
+        run: nix flake check --all-systems --no-build
+
+      - name: Build packages (x86_64-linux)
+        run: nix build .#vykar .#vykar-server .#vykar-gui --print-build-logs