ci: run tests on Linux, Windows and macOS; add non-blocking lint job

Tests previously ran on Ubuntu only, so Windows- and macOS-specific breakage went unseen. Add a 3-OS test matrix and a separate lint job for rustfmt and clippy. Lint is continue-on-error because the tree is not yet fmt-clean or clippy-clean.

Also add BEDIT characterization tests that pin current session, undo and refusal behavior ahead of the editor refactor. Locked tests guard behavior that must survive; documenting tests record questionable behavior expected to change.

.github/workflows/ci.yml

@@ -1,10 +1,8 @@
 name: Tests
 
-# Until now nothing ran the test suite on a change: the release workflows
-# build the app, web-check.yml type-checks the wasm target, and pull requests
-# reported their own local runs. Run the workspace tests on every push to
-# main and on every pull request. There is deliberately no formatter check:
-# the tree is not rustfmt-clean and is not going to be reformatted.
+# Runs the workspace tests on every push to main and on every pull request,
+# on Linux, Windows and macOS. A separate lint job reports rustfmt and clippy
+# results without blocking merges until the existing tree has been cleaned up.
 
 on:
   push:
@@ -20,13 +18,19 @@ concurrency:
 
 jobs:
   test:
-    runs-on: ubuntu-22.04
+    name: test (${{ matrix.os }})
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-22.04, windows-latest, macos-14]
     steps:
       - uses: actions/checkout@v4
 
       # The same libraries the Linux release build needs (windowing, GL,
       # fonts); the packaging tools are left out.
-      - name: System libraries
+      - name: System libraries (Linux)
+        if: runner.os == 'Linux'
         run: |
           sudo apt-get update
           sudo apt-get install -y \
@@ -37,8 +41,38 @@ jobs:
 
       - uses: Swatinem/rust-cache@v2
 
-      # LC_ALL=C: a few tests compare English message text.
+      # LC_ALL=C: a few tests compare English message text. The GPU-backed
+      # tests are #[ignore]d and need an adapter, so they do not run here.
       - name: cargo test
         env:
           LC_ALL: C
         run: cargo test --workspace --locked
+
+  lint:
+    name: lint (informational)
+    runs-on: ubuntu-22.04
+    # Non-blocking until the existing tree is cleaned up. The tree currently has
+    # thousands of rustfmt diffs and several clippy warnings and errors, so
+    # these checks report status without failing the workflow.
+    continue-on-error: true
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: System libraries
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y \
+            libgl1-mesa-dev libx11-dev libxcursor-dev libxi-dev libxrandr-dev \
+            libxkbcommon-dev libwayland-dev libfontconfig1-dev libfreetype6-dev
+
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          components: rustfmt, clippy
+
+      - uses: Swatinem/rust-cache@v2
+
+      - name: cargo fmt --check
+        run: cargo fmt --all -- --check
+
+      - name: cargo clippy
+        run: cargo clippy --workspace --all-targets --locked