Add CLAUDE.md with project guidance for Claude Code

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Commands
+
+**Run tests:**
+
+```bash
+julia --project -e "using Pkg; Pkg.test()"
+```
+
+**Run a single test file:**
+
+```bash
+julia --project test/positioning/test-psa.jl
+```
+
+**Format code (Runic.jl):**
+
+```bash
+pre-commit run runic --all-files
+```
+
+**Run all linting/formatting checks:**
+
+```bash
+pre-commit run -a
+```
+
+**Build docs:**
+
+```bash
+julia --project=docs docs/make.jl
+```
+
+## Architecture
+
+The package provides a unified interface to multiple solar position algorithms. Structure:
+
+```text
+src/
+  SolarPosition.jl       # Main module, re-exports all submodules
+  Positioning/           # Solar position algorithms
+    Positioning.jl       # Observer struct, SolPos/ApparentSolPos types, solar_position() API
+    psa.jl               # PSA algorithm (default, ±0.0083°)
+    noaa.jl, spa.jl,     # Other algorithms (NOAA, SPA, Walraven, USNO)
+    walraven.jl, usno.jl
+    deltat.jl            # Delta T / leap seconds
+  Refraction/            # Atmospheric refraction correction models
+    Refraction.jl        # Abstract base + interface
+    hughes.jl, bennett.jl, sg2.jl, spa.jl, ...
+  Utilities/             # Sunrise/sunset/transit calculations
+    srt.jl, spa.jl
+ext/                     # Weak dependency extensions
+  SolarPositionMakieExt.jl          # Sun path plotting
+  SolarPositionOhMyThreadsExt.jl    # Parallel solar position computation
+  SolarPositionModelingToolkitExt.jl # Symbolic models for MTK
+```
+
+**Core API pattern:** `solar_position(algorithm, observer, datetime)` returns a `SolPos` or `ApparentSolPos` (if refraction is included). The `Observer` struct holds location (lat/lon/altitude) and optional atmospheric parameters (pressure, temperature).
+
+**Test discovery:** Test files matching `test-*.jl` under `test/` are automatically discovered and wrapped in `@testset`. Reference values live in `expected-values.jl` files alongside algorithm tests.
+
+## Code Style
+
+- Formatter: **Runic.jl** (enforced via pre-commit). Run before committing.
+- Imports: All used symbols must be explicitly imported (checked by ExplicitImports.jl).
+- Package quality is checked with **Aqua.jl** and type inference with **JET.jl** (Julia 1.12 only).