ci: add Rust checker workflow (fmt + clippy -D warnings + test)

Mirrors spec-check's framing: an internal health gate on the implementation
(build/test/lint), not a spec→impl conformance gate (alignment is by review).
Installs fonts-noto-cjk ahead of the typst-compiling crates (WU-4+), since the
engineering files are CJK-heavy.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/checker-check.yml

@@ -0,0 +1,53 @@
+name: checker check
+
+# Builds and lints the Rust implementation crates under crates/ (the rule-based
+# checker that "stands in Lean's position" at product runtime).
+#
+# Like spec-check, this is an INTERNAL gate on the implementation's own health
+# (does it build, pass its tests, satisfy clippy + rustfmt?). It is NOT a
+# spec-to-implementation conformance gate — implementations align to the Lean
+# contract by human review, not by CI. See the repo README.
+
+on:
+  push:
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  checker-check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+
+      - name: Install Rust toolchain (stable + clippy + rustfmt)
+        uses: dtolnay/rust-toolchain@stable
+        with:
+          components: clippy, rustfmt
+
+      - name: Cache cargo registry + build
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            target
+          key: cargo-${{ runner.os }}-${{ hashFiles('Cargo.lock') }}
+          restore-keys: |
+            cargo-${{ runner.os }}-
+
+      # Fonts for typst compilation (WU-4 onward): the engineering files are
+      # CJK-heavy, so the compile-check / build tests need a CJK font present.
+      # Harmless before those crates exist.
+      - name: Install CJK fonts
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y fonts-noto-cjk
+
+      - name: Format check
+        run: cargo fmt --all --check
+
+      - name: Clippy (deny warnings)
+        run: cargo clippy --all-targets --all-features -- -D warnings
+
+      - name: Test
+        run: cargo test --all-features --workspace