Add Doxygen, clang-format, cppcheck, and Gitea CI

Doxygen:
- Doxyfile: minimal config, HTML output to docs/, no LaTeX
- @file/@brief on all source files, full @param/@return on public API
- docs/ added to .gitignore

clang-format (14, Linux brace style, 4-space, column 100):
- .clang-format added
- Applied to entire codebase; this commit is the canonical baseline
- just format rewrites in-place; just format-check is the CI gate

cppcheck (--enable=warning,style,performance,portability):
- Linker-symbol pointer comparisons in startup.c suppressed with
  inline cppcheck-suppress (false positives, not real bugs)
- just lint runs cppcheck; zero warnings required to pass

Dockerfile gains clang-format, cppcheck, doxygen packages so all
tools run inside the existing container -- host stays clean.

Gitea Actions (.gitea/workflows/ci.yml):
- Four parallel jobs: build, format, lint, docs
- All jobs use the same Dockerfile-based image
- Doxygen job fails on any warning line in output

.gitea/workflows/ci.yml

@@ -0,0 +1,99 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+env:
+  IMAGE: ptt-builder
+
+jobs:
+  build:
+    name: Build firmware
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          submodules: recursive
+
+      - name: Build container image
+        run: docker build -t $IMAGE .
+
+      - name: Compile firmware
+        run: |
+          docker run --rm \
+            --user "$(id -u):$(id -g)" \
+            -v "$PWD:/src" \
+            $IMAGE \
+            sh -c "cmake -B /src/build -G Ninja \
+                         -DCMAKE_BUILD_TYPE=MinSizeRel /src \
+                   && ninja -C /src/build"
+
+      - name: Print size
+        run: |
+          docker run --rm \
+            -v "$PWD:/src" \
+            $IMAGE \
+            arm-none-eabi-size /src/build/firmware
+
+  format:
+    name: Check formatting
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Build container image
+        run: docker build -t $IMAGE .
+
+      - name: clang-format check
+        run: |
+          docker run --rm \
+            --user "$(id -u):$(id -g)" \
+            -v "$PWD:/src" \
+            $IMAGE \
+            sh -c "find /src/src /src/include -name '*.c' -o -name '*.h' | \
+                   xargs clang-format --dry-run --Werror \
+                         --style=file:/src/.clang-format"
+
+  lint:
+    name: Static analysis
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Build container image
+        run: docker build -t $IMAGE .
+
+      - name: cppcheck
+        run: |
+          docker run --rm \
+            --user "$(id -u):$(id -g)" \
+            -v "$PWD:/src" \
+            $IMAGE \
+            sh -c "cppcheck --error-exitcode=1 \
+                            --enable=warning,style,performance,portability \
+                            --suppress=missingInclude \
+                            --inline-suppr \
+                            --std=c11 \
+                            -I /src/include \
+                            /src/src/"
+
+  docs:
+    name: Build documentation
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Build container image
+        run: docker build -t $IMAGE .
+
+      - name: Doxygen
+        run: |
+          docker run --rm \
+            --user "$(id -u):$(id -g)" \
+            -v "$PWD:/src" \
+            $IMAGE \
+            sh -c "cd /src && doxygen Doxyfile 2>&1 | tee /tmp/doxy.log && \
+                   ! grep -q 'warning:' /tmp/doxy.log"