Add the Apache 2.0 licence text and CI

Cargo.toml has declared Apache-2.0 without the licence being in the tree.

CI checks formatting, clippy and the tests on every push, with warnings as
errors. The workflow lives in .github/workflows, which Gitea Actions also
reads.

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

.github/workflows/ci.yml

@@ -0,0 +1,28 @@
+# Runs on every push and pull request. Gitea Actions reads this file too
+# (it falls back to .github/workflows), so one definition serves both.
+name: ci
+
+on:
+  push:
+  pull_request:
+
+env:
+  CARGO_TERM_COLOR: always
+  # Warnings fail the build here, and only here: locally they stay warnings
+  # so that work in progress compiles.
+  RUSTFLAGS: -D warnings
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          components: rustfmt, clippy
+      - name: Format
+        run: cargo fmt --all -- --check
+      - name: Lint
+        run: cargo clippy --workspace --all-targets -- -D warnings
+      - name: Test
+        run: cargo test --workspace