v0.2.0 transitioned to api-only, extended and improved features, docs and tests added

doc/architecture.md

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+# Architecture
+
+## Components
+
+### Transport Layer (`internal/http`)
+Fiber web server handling HTTP requests/responses. Implements routing, rate limiting, content-type validation, request parsing. Translates HTTP to internal Command objects.
+
+### Processing Layer (`internal/processor`)
+Central command handler containing business logic. Single `Execute(Command)` entry point decouples transport from logic. Uses synchronous UCI engine for validation, asynchronous EngineQueue for computer moves.
+
+### Service Layer (`internal/service`)
+In-memory state storage without chess logic. Thread-safe game map protected by RWMutex. Manages game lifecycle, snapshots, and player configuration.
+
+### Supporting Modules
+- **Engine** (`internal/engine`): UCI protocol wrapper for Stockfish process communication
+- **Game** (`internal/game`): Game state with snapshot history
+- **Board** (`internal/board`): FEN parsing and ASCII generation
+- **Core** (`internal/core`): Shared types, API models, error constants
+
+## Request Flow
+
+### Human Move
+1. HTTP handler receives `POST /games/{id}/moves` with UCI move
+2. Creates MakeMoveCommand, calls `processor.Execute()`
+3. Processor validates move via locked validation engine
+4. If legal, gets new FEN from engine
+5. Calls `service.ApplyMove()` to update state
+6. Returns GameResponse
+
+### Computer Move
+1. HTTP handler receives `POST /games/{id}/moves` with `{"move": "cccc"}`
+2. Processor sets game state to `pending`
+3. Submits task to EngineQueue, returns immediately
+4. Worker goroutine calculates move with dedicated Stockfish instance
+5. Callback updates game state via service
+6. Client polls for completion
+
+## Concurrency
+
+- **HTTP Server**: Fiber handles concurrent connections
+- **Game State**: Single RWMutex protects game map (concurrent reads, serial writes)
+- **Engine Workers**: Fixed pool (2 workers) with dedicated Stockfish processes
+- **Validation Engine**: Single mutex-protected instance for synchronous validation
+
+## Data Structures
+
+### Game Snapshot
+```go
+type Snapshot struct {
+    FEN           string
+    PreviousMove  string
+    NextTurnColor Color
+    PlayerID      string
+}
+```
+
+### Command Pattern
+Commands encapsulate operations with type and arguments, processed by single Execute method.
+
+### Player Configuration
+Players identified by UUID, configured with type (human/computer), skill level, and search time.