ci: run the workspace tests on every change; drop the unused flake

- .github/workflows/ci.yml runs `cargo test --workspace --locked` on
  pushes to main and on pull requests, with the Linux libraries the
  release build installs. No formatter check: the tree is not
  rustfmt-clean and stays that way.
- flake.nix, flake.lock and the direnv hook are removed. Nothing used
  them, and their pinned dependency hashes were already stale after the
  codec and kernel bumps.
- Under cfg(test) the "system" language resolves to en-US. The unit
  tests compare English text through one process-wide loader, and every
  apply_config of a default config asked for the system language, so a
  Turkish locale failed 22 tests and parallel tests raced each other
  through a language switch (the thickness-row test found its section
  under one language and looked it up under another).
- The arc grip test follows #1352: the start, end and midpoint grips
  drive the whole arc.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -0,0 +1,44 @@
+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.
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: tests-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  test:
+    runs-on: ubuntu-22.04
+    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
+        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
+
+      - uses: Swatinem/rust-cache@v2
+
+      # LC_ALL=C: a few tests compare English message text.
+      - name: cargo test
+        env:
+          LC_ALL: C
+        run: cargo test --workspace --locked