feat: implement Phase 1 MVP foundation

[thompcd]

This commit implements steps 1-7 of the BenchLibrary roadmap:

Step 1 - Monorepo Structure:

• Created BenchLibrary.sln solution file
• Added .editorconfig with C# coding standards
• Created src/, tests/, samples/, .github/workflows/ directories

Step 2 - Central Package Management:

• Created Directory.Build.props with .NET 8, C# 12, nullable types
• Created Directory.Packages.props with centralized NuGet versions

Step 3 - BenchLibrary.Core:

• IInstrument interface for test instruments
• ITestRepository interface for data access
• Measurement, TestResult, DeviceUnderTest models
• TestStatus, InstrumentType, MeasurementUnit enums
• Full XML documentation on all public APIs

Step 4 - BenchLibrary.SixSigma:

• StatisticsCalculator for basic stats (mean, stddev, range)
• CapabilityCalculator for Cp, Cpk, Pp, Ppk calculations
• ControlChartCalculator for X-bar, R, I-MR charts
• ParetoCalculator for 80/20 analysis
• SigmaLevelCalculator for DPMO and sigma level
• Supporting models with capability ratings

Step 5 - Unit Tests:

• BenchLibrary.Core.Tests with model tests
• BenchLibrary.SixSigma.Tests with calculation tests
• Uses xUnit, FluentAssertions, Moq

Step 6 - BenchLibrary.Data:

• BenchLibraryDbContext with EF Core
• TestRepository implementation
• DeviceRepository implementation
• SQLite and PostgreSQL support
• DI extension methods

Step 7 - BenchLibrary.Web:

• Blazor Server application with MudBlazor
• Home page with feature overview
• SPC Dashboard with X-bar/R chart simulation
• Capability Analysis page with Cpk visualization
• Pareto Analysis page with 80/20 breakdown
• Sigma Level Calculator page
• Dark mode support
• Responsive layout