Add test coverage reporting and refactor CI configuration

Added comprehensive test coverage reporting with HPC (Haskell Program
Coverage) that generates both text and HTML coverage reports. Coverage
runs in a separate CI job to avoid interfering with regular build
caching.

Coverage reports include:
- Text summary with expression, boolean, alternative, and declaration
  coverage percentages
- HTML reports with line-by-line highlighting
- Automatic upload as GitHub Actions artifacts

Refactored CI configuration into modular structure with auto-discovery:
- Split monolithic ci.nix into ci/ directory
- Separate files for each workflow (check.nix, pages.nix)
- Automatic discovery of workflow files in ci/ directory
- Easier to add new workflows by simply adding files

Updated .gitignore to exclude coverage artifacts (*.tix, .hpc/,
coverage/).

Author: typedrat

.github/workflows/check.yml

@@ -21,6 +21,27 @@ jobs:
         uses: DeterminateSystems/nix-installer-action@v21
       - name: Run flake check
         run: nix flake check
+  coverage:
+    name: Test coverage
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@v21
+      - name: Run tests with coverage
+        run: |
+          nix develop -c cabal test --enable-coverage
+          TIX_FILE=$(find dist-newstyle -name "terranix-codegen-tests.tix" -type f | head -1)
+          MIX_DIRS=$(find dist-newstyle -type d -name "mix" -path "*/hpc/vanilla/mix" | sed "s/^/--hpcdir=/")
+          mkdir -p coverage
+          nix develop -c hpc report $MIX_DIRS "$TIX_FILE" | tee coverage/coverage.txt
+          nix develop -c hpc markup $MIX_DIRS --destdir=coverage "$TIX_FILE"
+      - name: Upload coverage report
+        uses: actions/upload-artifact@v4
+        with:
+          name: coverage-report
+          path: coverage/
 name: Check
 "on":
   pull_request: {}

.gitignore

@@ -4,6 +4,11 @@ dist-*
 cabal.project.local
 cabal.project.local~
 
+# coverage
+*.tix
+.hpc
+coverage/
+
 # nix
 result
 result-*

nix/flake/ci.nix

@@ -0,0 +1,92 @@
+_: {
+  perSystem = _: {
+    githubActions.workflows.check = {
+      name = "Check";
+
+      on = {
+        push = {};
+        pullRequest = {};
+      };
+
+      permissions = {
+        contents = "read";
+        id-token = "write";
+      };
+
+      defaults.job = {
+        runsOn = "ubuntu-latest";
+      };
+
+      jobs = {
+        check = {
+          name = "Run flake check";
+          steps = [
+            {
+              name = "Checkout";
+              uses = "actions/checkout@v4";
+            }
+            {
+              name = "Install Nix";
+              uses = "DeterminateSystems/nix-installer-action@v21";
+            }
+            {
+              name = "Run flake check";
+              run = "nix flake check";
+            }
+          ];
+        };
+
+        build = {
+          name = "Build package";
+          steps = [
+            {
+              name = "Checkout";
+              uses = "actions/checkout@v4";
+            }
+            {
+              name = "Install Nix";
+              uses = "DeterminateSystems/nix-installer-action@v21";
+            }
+            {
+              name = "Build package";
+              run = "nix build";
+            }
+          ];
+        };
+
+        coverage = {
+          name = "Test coverage";
+          steps = [
+            {
+              name = "Checkout";
+              uses = "actions/checkout@v4";
+            }
+            {
+              name = "Install Nix";
+              uses = "DeterminateSystems/nix-installer-action@v21";
+            }
+            {
+              name = "Run tests with coverage";
+              run = ''
+                nix develop -c cabal test --enable-coverage
+                TIX_FILE=$(find dist-newstyle -name "terranix-codegen-tests.tix" -type f | head -1)
+                MIX_DIRS=$(find dist-newstyle -type d -name "mix" -path "*/hpc/vanilla/mix" | sed "s/^/--hpcdir=/")
+                mkdir -p coverage
+                nix develop -c hpc report $MIX_DIRS "$TIX_FILE" | tee coverage/coverage.txt
+                nix develop -c hpc markup $MIX_DIRS --destdir=coverage "$TIX_FILE"
+              '';
+            }
+            {
+              name = "Upload coverage report";
+              uses = "actions/upload-artifact@v4";
+              with_ = {
+                name = "coverage-report";
+                path = "coverage/";
+              };
+            }
+          ];
+        };
+      };
+    };
+  };
+}

nix/flake/ci/check.nix

@@ -0,0 +1,34 @@
+{inputs, ...}: {
+  imports = let
+    # Auto-discover all workflow files in this directory
+    workflowFiles = builtins.filter (name: name != "default.nix") (
+      builtins.attrNames (builtins.readDir ./.)
+    );
+  in
+    [
+      inputs.files.flakeModules.default
+      inputs.github-actions-nix.flakeModules.default
+    ]
+    ++ map (name: ./${name}) workflowFiles;
+
+  perSystem = {
+    config,
+    lib,
+    ...
+  }: {
+    files.files = let
+      go = name: drv: {
+        path_ = ".github/workflows/${name}";
+        inherit drv;
+      };
+    in
+      lib.mapAttrsToList go config.githubActions.workflowFiles;
+
+    apps.write-files = {
+      type = "app";
+      program = config.files.writer.drv;
+    };
+
+    githubActions.enable = true;
+  };
+}