Add project documentation and conventions

CLAUDE.md

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+# Advent of Code - C# Solutions
+
+## Stack
+
+- .NET 9.0, C# latest, Autofac DI
+- NUnit 4.5 (test framework), NUnit3TestAdapter, Microsoft.NET.Test.Sdk, coverlet.collector
+
+## Build & Test
+
+```bash
+dotnet build AdventOfCode.sln
+dotnet test AdventOfCodeTests/AdventOfCodeTests.csproj
+dotnet test --filter "FullyQualifiedName~Day5Tests"   # run specific test class
+dotnet run --project AdventOfCode 2024 15             # run specific day
+```
+
+## Architecture
+
+- `IDay` interface: `Day`, `Year`, `Part1(string path)`, `Part2(string path)` — returns `string`
+- `Framework/DayBase` abstract class reads file lines, delegates to `Part1(IEnumerable<string>)` / `Part2(IEnumerable<string>)`
+- Each year has its own `DayBase` that sets `Year` (e.g., `Events/_2024/Days/DayBase.cs`)
+- Autofac auto-discovers all `IDay` implementations via assembly scanning in `Program.cs`
+
+## File Layout
+
+| Item | Path |
+|---|---|
+| Solution class | `AdventOfCode/Events/{YEAR}/Days/Day{N}.cs` |
+| Year DayBase | `AdventOfCode/Events/{YEAR}/Days/DayBase.cs` |
+| Input files | `AdventOfCode/Events/{YEAR}/InputFiles/{N}.txt` |
+| Test class | `AdventOfCodeTests/Events/{YEAR}/Days/Day{N}Tests.cs` |
+| Test data | `AdventOfCodeTests/Events/{YEAR}/TestData/{N}.txt` |
+| Common utils | `AdventOfCode/Common/` (Coordinate, Direction, Vector) |
+
+## Conventions
+
+- Namespaces: `AdventOfCode.Events._{YEAR}.Days` (underscore prefix for numeric year)
+- Class names: `Day1`, `Day12` (no leading zeros)
+- Test classes: `Day1Tests`, `Day12Tests` with `[TestFixture]` attribute
+- Tests instantiate the day directly: `private readonly DayNN _systemUnderTest = new();`
+- Test data files use letter suffixes for variants: `12a.txt`, `12b.txt`
+- Input/test data files must be added to `.csproj` with `CopyToOutputDirectory: PreserveNewest`
+- Both Part1 and Part2 return answers as `string`