Add .clang-format file for consistent code formatting

This also adds a CI job to impose the rules on new code.

Author: deckstose

.clang-format

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+---
+BasedOnStyle: LLVM
+Language: Cpp
+
+AlignAfterOpenBracket: BlockIndent
+AllowShortFunctionsOnASingleLine: Inline
+ColumnLimit:     120
+Cpp11BracedListStyle: false
+IncludeBlocks: Regroup
+IncludeCategories:
+  - Regex: '^".*"$'
+    Priority: 1
+  - Regex: '^<.*\..*>$'
+    Priority: 2
+  - Regex: '^<.*>$'
+    Priority: 3
+IndentPPDirectives: AfterHash
+NamespaceIndentation: All
+QualifierAlignment: Left
+PointerAlignment: Left
+SpaceBeforeCpp11BracedList: true
+TabWidth:        2
+UseTab:          Always
+...

.github/workflows/clang-format.yml

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+name: Clang Format
+
+on:
+  pull_request:
+    branches: main
+    paths:
+      - 'src/**pp'
+  push:
+    branches: main
+    paths:
+      - 'src/**pp'
+
+jobs:
+  check:
+    name: Check style
+    runs-on: ubuntu-24.04
+    container: alpine:edge
+    concurrency:
+      group: ${{ github.workflow }}-${{ github.pull_request.number || github.ref }}
+      cancel-in-progress: true
+    steps:
+      - name: Install clang-format
+        run: apk add --no-cache clang-extra-tools git python3
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - name: Add safe directory
+        run: git config --global --add safe.directory $GITHUB_WORKSPACE
+      - name: Check code style
+        run: git clang-format --diff --extensions cpp,hpp ${{ github.event.pull_request.base.sha || github.event.before }}