Document architecture and add CI, formatting, and GPL license

Author: watsonryan

.clang-format

@@ -0,0 +1,4 @@
+BasedOnStyle: LLVM
+IndentWidth: 2
+ColumnLimit: 110
+SortIncludes: Never

.clang-tidy

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+Checks: '-*,bugprone-*,performance-*,readability-*'
+WarningsAsErrors: ''
+HeaderFilterRegex: 'include/.*|tests/.*|examples/.*'

.github/workflows/ci.yml

@@ -0,0 +1,39 @@
+name: ci
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  debug:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Configure
+        run: cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Debug
+      - name: Build
+        run: cmake --build build -j
+      - name: Test
+        run: ctest --test-dir build --output-on-failure
+
+  asan:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Configure
+        run: cmake -S . -B build-asan -G Ninja -DCMAKE_BUILD_TYPE=Debug -DODE_ENABLE_ASAN=ON
+      - name: Build
+        run: cmake --build build-asan -j
+      - name: Test
+        run: ctest --test-dir build-asan --output-on-failure
+
+  ubsan:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Configure
+        run: cmake -S . -B build-ubsan -G Ninja -DCMAKE_BUILD_TYPE=Debug -DODE_ENABLE_UBSAN=ON
+      - name: Build
+        run: cmake --build build-ubsan -j
+      - name: Test
+        run: ctest --test-dir build-ubsan --output-on-failure

LICENSE

@@ -0,0 +1,11 @@
+GNU GENERAL PUBLIC LICENSE
+Version 3, 29 June 2007
+
+Copyright (C) 2026 Watson
+
+This project is licensed under the GNU General Public License, version 3.
+You may obtain a copy of the full license text at:
+
+https://www.gnu.org/licenses/gpl-3.0.txt
+
+SPDX-License-Identifier: GPL-3.0-only

README.md

@@ -8,6 +8,23 @@ C++20 explicit Runge-Kutta integrator for non-stiff ODEs:
 - RKF78 (embedded adaptive 7(8), fixed-step using high-order solution also supported)
 - RK8 alias (fixed-step using RKF78 high-order weights)
 
+## Architecture
+
+```mermaid
+flowchart TD
+  A[Client code] --> B[integrate API]
+  B --> C[Runtime method dispatch]
+  B --> D[Compile-time tableau API]
+  C --> E[Drivers]
+  D --> E
+  E --> F[ExplicitRKStepper]
+  F --> G[RK4 tableau]
+  F --> H[RKF45 tableau]
+  F --> I[RKF78 tableau]
+  E --> J[Error norm and step-size controller]
+  E --> K[Integration result and stats]
+```
+
 ## Build and test
 
 ```bash
@@ -16,10 +33,48 @@ cmake --build --preset macos-debug -j
 ctest --preset macos-debug --output-on-failure
 ```
 
-## API quick start
+## Sanitizer runs
+
+AddressSanitizer:
+
+```bash
+cmake --preset macos-asan
+cmake --build --preset macos-asan -j
+ctest --preset macos-asan --output-on-failure
+```
+
+UndefinedBehaviorSanitizer:
+
+```bash
+cmake --preset macos-ubsan
+cmake --build --preset macos-ubsan -j
+ctest --preset macos-ubsan --output-on-failure
+```
+
+## Install and package consumption
+
+```bash
+cmake --preset macos-release
+cmake --build --preset macos-release -j
+cmake --install build/macos-release --prefix /tmp/ode-install
+```
+
+Downstream CMake usage:
+
+```cmake
+find_package(ode CONFIG REQUIRED)
+target_link_libraries(your_target PRIVATE ode::ode)
+```
+
+The repository includes a package consumer smoke test (`ode_package_install_smoke`).
+
+## API usage
+
+Runtime method selection:
 
 ```cpp
 #include <ode/ode.hpp>
+#include <vector>
 
 using State = std::vector<double>;
 State y0{1.0};
@@ -36,3 +91,48 @@ opt.atol = 1e-12;
 
 auto res = ode::integrate(ode::RKMethod::RKF78, rhs, 0.0, y0, 1.0, opt);
 ```
+
+Compile-time tableau selection:
+
+```cpp
+#include <ode/integrate_method.hpp>
+#include <ode/tableaus/rkf45.hpp>
+
+auto res = ode::integrate_with_tableau<ode::TableauRKF45>(rhs, t0, y0, t1, opt);
+```
+
+## Simple 2-body orbital example
+
+Build and run:
+
+```bash
+cmake --preset macos-debug
+cmake --build --preset macos-debug -j
+./build/macos-debug/ode_two_body_example
+```
+
+This integrates a circular LEO two-body problem for approximately one orbital period and prints final state and step stats.
+
+## Profiling
+
+```bash
+tools/profile.sh
+```
+
+Optional overrides:
+- `ODE_PERF_SAMPLES`
+- `ODE_PERF_ITERATIONS`
+
+## API docs (Doxygen)
+
+```bash
+cmake --preset macos-debug -DODE_BUILD_DOCS=ON
+cmake --build --preset macos-debug --target ode_docs
+```
+
+Generated HTML is written under `build/<preset>/docs/html/`.
+
+## License
+
+This project is licensed under the GNU General Public License v3.0.
+See `LICENSE`.