docs: add ADR-002 through ADR-010 covering architecture, persistence, frontend, and infrastructure decisions

[Copilot]

Continues the ADR authoring session. ADR-001 (Clean Architecture) existed; this adds the remaining nine decisions across all four architectural themes.

ADRs Added

Architecture & Design

• ADR-002 — CQRS via MediatR: documents ICommand/IQuery abstractions, the 14 commands and 4 queries, pipeline behavior extensibility, and rules prohibiting repository injection in controllers
• ADR-003 — Specification Pattern: documents ISpecification<T> usage and explicitly flags GetBySpecificationAsync in CosmosDbGameRepository as technical debt — it issues SELECT * FROM c and filters in-memory; direct SQL query methods should be preferred until a Cosmos DB expression translator is built

Persistence

• ADR-004 — Cosmos DB as canonical IGameRepository: retires AzureBlobGameRepository from game persistence, deprecates the UseCosmosDb env var toggle, documents partition key strategy
• ADR-005 — InMemoryPuzzleRepository scoped to puzzle generation: documents the CircularStack<SudokuPuzzle>(100) design and the intentional NotSupportedException on CreateAsync/LoadAllAsync — prevents contributors from "fixing" it by wiring it to a database
• ADR-006 — Blob storage retirement from game state: Sudoku.Storage.Azure is retained but AzureBlobGameRepository must not be re-registered as IGameRepository; lists candidate repurposing use cases pending a follow-up ADR

Frontend

• ADR-007 — React/Vite as strategic UI target: Sudoku.Blazor is maintenance-only; defines explicit exit criteria for Blazor retirement (feature parity + user migration)

Infrastructure

• ADR-008 — Aspire for service orchestration: documents the three-service composition (sudoku-api, sudoku-blazor, sudoku-react), environment variable injection contract, and why Docker Compose was not chosen
• ADR-009 — Azure App Configuration: documents the three connection modes (connection string, Managed Identity, DefaultAzureCredential fallback), key/label filtering strategy, and the rule that App Configuration holds no secrets
• ADR-010 — Azure Key Vault: establishes Key Vault as the exclusive secret store, documents Managed Identity as the required auth model for Azure-hosted environments, enforces no secrets in appsettings.json or App Configuration