Add package-level doc comments for go doc and gopls

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

doc.go

@@ -0,0 +1,39 @@
+// Package httpx provides a high-level HTTP client with composable middleware
+// for retry, circuit breaking, load balancing, structured logging, and more.
+//
+// The client is configured via functional options and assembled as a middleware
+// chain around a standard http.RoundTripper:
+//
+//	Logging → User Middlewares → Retry → Circuit Breaker → Balancer → Transport
+//
+// # Quick start
+//
+//	client := httpx.New(
+//	    httpx.WithBaseURL("https://api.example.com"),
+//	    httpx.WithTimeout(10 * time.Second),
+//	    httpx.WithRetry(),
+//	    httpx.WithCircuitBreaker(),
+//	)
+//	defer client.Close()
+//
+//	resp, err := client.Get(ctx, "/users/1")
+//
+// # Request builders
+//
+// NewJSONRequest and NewFormRequest create requests with appropriate
+// Content-Type headers and GetBody set for retry compatibility.
+//
+// # Error handling
+//
+// Failed requests return *httpx.Error with structured fields (Op, URL,
+// StatusCode). Sentinel errors ErrRetryExhausted, ErrCircuitOpen, and
+// ErrNoHealthy can be checked with errors.Is.
+//
+// # Sub-packages
+//
+//   - middleware — client-side middleware (logging, auth, headers, recovery, request ID)
+//   - retry — configurable retry with backoff and Retry-After support
+//   - circuitbreaker — per-host circuit breaker (closed → open → half-open)
+//   - balancer — client-side load balancing with health checking
+//   - server — production HTTP server with router, middleware, and graceful shutdown
+package httpx