bench: add RenderToString suite covering the string-returning API

source/Handlebars.Benchmark/RenderToString.cs

@@ -0,0 +1,62 @@
+using System.Collections.Generic;
+using System.IO;
+using BenchmarkDotNet.Attributes;
+using HandlebarsDotNet;
+
+namespace HandlebarsNet.Benchmark
+{
+    // Render time for the string-returning template API — the most common way the
+    // library is consumed. Unlike the other Render* benchmarks (which write to
+    // TextWriter.Null and therefore only measure resolution overhead), this suite
+    // pays the real output cost: HTML encoding and StringBuilder-backed writes.
+    //
+    // Content=clean:  values contain nothing that needs HTML escaping (typical data).
+    // Content=html:   values are dense with characters that must be escaped (worst case).
+    [MemoryDiagnoser]
+    public class RenderToString
+    {
+        private HandlebarsTemplate<object, object> _template;
+        private object _data;
+
+        private const int ItemCount = 50;
+
+        private const string Source =
+            "<ul>" +
+            "{{#each items}}" +
+            "<li id=\"item-{{@index}}\"><span>{{name}}</span> &mdash; {{description}} " +
+            "<em>x{{qty}}</em>{{#if onSale}} <strong>SALE</strong>{{/if}}</li>" +
+            "{{/each}}" +
+            "</ul>";
+
+        [Params("clean", "html")]
+        public string Content { get; set; }
+
+        [GlobalSetup]
+        public void Setup()
+        {
+            var handlebars = Handlebars.Create();
+            _template = handlebars.Compile(Source);
+
+            var description = Content == "clean"
+                ? "Reliable everyday item for home and office use"
+                : "Fast & reliable <everyday> item \"for\" home & office use";
+
+            var items = new List<object>(ItemCount);
+            for (var i = 0; i < ItemCount; i++)
+            {
+                items.Add(new
+                {
+                    name = $"Product {i:D4}",
+                    description,
+                    qty = i * 7 + 1,
+                    onSale = i % 3 == 0
+                });
+            }
+
+            _data = new { items };
+        }
+
+        [Benchmark]
+        public string Render() => _template(_data);
+    }
+}