Fix prepared script performance regression from shared identifier cache thrashing (#2324)

When using Engine.PrepareScript(), the AstAnalyzer stored JintIdentifierExpression instances
in AST node UserData. Since JintExpression.Build() returns these cached instances directly,
ALL references to the same identifier across all scopes and all engine instances shared a single
JintIdentifierExpression with its mutable _cachedEnvironment field. Different scopes continuously
overwrote each other's cached environment, causing the fast-path lookup to always miss.

The fix stores only the immutable BindingName in UserData instead. Each engine execution now
creates fresh JintIdentifierExpression instances (with independent environment caches) while
still reusing the pre-computed BindingName to avoid repeated JsString/BindingName allocation.

This also fixes a minor bug in the single-parameter JintIdentifierExpression constructor that
created two redundant BindingName instances (one in the this() chain, one in the body).

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: lahma

Jint.Benchmark/Jint.Benchmark.csproj

@@ -21,6 +21,7 @@
   </PropertyGroup>
   <ItemGroup>
     <None Include=".\Scripts\*.*" CopyToOutputDirectory="PreserveNewest" />
+    <None Include=".\Scripts\template-rendering\**\*.*" CopyToOutputDirectory="PreserveNewest" LinkBase="Scripts\template-rendering" />
     <None Include="..\Jint.Tests.CommonScripts\Scripts\**" CopyToOutputDirectory="PreserveNewest" LinkBase="Scripts" />
   </ItemGroup>
   <ItemGroup>

Jint.Benchmark/PreparedScriptBenchmark.cs

@@ -0,0 +1,71 @@
+using BenchmarkDotNet.Attributes;
+
+namespace Jint.Benchmark;
+
+/// <summary>
+/// Replicates the exact pattern from JavaScriptEngineSwitcher's JsExecutionHeavyBenchmark:
+/// a Handlebars template rendering library is loaded on 4 separate engine instances,
+/// each calling renderTemplate() with different content items.
+/// See: https://github.com/Taritsyn/JavaScriptEngineSwitcher/blob/master/test/JavaScriptEngineSwitcher.Benchmarks/JsExecutionHeavyBenchmark.cs
+/// </summary>
+[MemoryDiagnoser]
+public class PreparedScriptBenchmark
+{
+    private string _libraryCode = null!;
+    private Prepared<Script> _preparedLibrary;
+    private ContentItem[] _contentItems = null!;
+
+    [GlobalSetup]
+    public void Setup()
+    {
+        var handlebars = File.ReadAllText("Scripts/template-rendering/lib/handlebars.js");
+        var helpers = File.ReadAllText("Scripts/template-rendering/lib/helpers.js");
+        _libraryCode = handlebars + "\n" + helpers;
+        _preparedLibrary = Engine.PrepareScript(_libraryCode);
+
+        var contentDir = "Scripts/template-rendering/content";
+        string[] names = ["hello-world", "contacts", "js-engines", "web-browser-family-tree"];
+        _contentItems = names.Select(name => new ContentItem(
+            File.ReadAllText(Path.Combine(contentDir, name, "template.handlebars")),
+            File.ReadAllText(Path.Combine(contentDir, name, "data.json"))
+        )).ToArray();
+    }
+
+    [Benchmark(Baseline = true)]
+    public void ExecuteStringOnMultipleEngines()
+    {
+        RenderTemplates(withPrecompilation: false);
+    }
+
+    [Benchmark]
+    public void ExecutePreparedOnMultipleEngines()
+    {
+        RenderTemplates(withPrecompilation: true);
+    }
+
+    private void RenderTemplates(bool withPrecompilation)
+    {
+        // First engine: load library and render first item
+        var engine = new Engine();
+        if (withPrecompilation)
+            engine.Execute(_preparedLibrary);
+        else
+            engine.Execute(_libraryCode);
+
+        engine.Invoke("renderTemplate", _contentItems[0].Template, _contentItems[0].Data);
+
+        // Remaining engines: each loads the same library and renders one item
+        for (int i = 1; i < _contentItems.Length; i++)
+        {
+            engine = new Engine();
+            if (withPrecompilation)
+                engine.Execute(_preparedLibrary);
+            else
+                engine.Execute(_libraryCode);
+
+            engine.Invoke("renderTemplate", _contentItems[i].Template, _contentItems[i].Data);
+        }
+    }
+
+    private sealed record ContentItem(string Template, string Data);
+}

Jint.Benchmark/Scripts/template-rendering/content/contacts/data.json

@@ -0,0 +1,20 @@
+{
+  "lastName": "Пупкин",
+  "firstName": "Василий",
+  "patronymic": "Иванович",
+  "address": {
+    "postalCode": 392032,
+    "country": "Россия",
+    "state": "Тамбовская область",
+    "city": "Тамбов",
+    "street": "Магистральная",
+    "houseNumber": "41к7",
+    "floor": 8,
+    "apartmentNumber": 115
+  },
+  "homePhone": "+7 (4752) 555-55-55",
+  "mobilePhone": "+7 (999) 689-99-99",
+  "email": "pupkin@mail.ru",
+  "website": "http:\/\/pupkin.narod.ru",
+  "icq": 698426795
+}

Jint.Benchmark/Scripts/template-rendering/content/contacts/template.handlebars

@@ -0,0 +1,13 @@
+<address>
+	<strong><abbr title="Фамилия Имя Отчество">Ф.И.О.</abbr>:</strong> {{lastName}} {{firstName}} {{patronymic}}<br />
+	<strong>Адрес:</strong>
+	{{#with address}}
+		{{postalCode}}, {{country}}, {{state}}, {{city}}, ул. {{street}}, д. {{houseNumber}}, кв. {{apartmentNumber}}
+	{{/with}}
+	<br />
+	<strong>Дом. тел.:</strong> {{homePhone}}<br />
+	<strong>Моб. тел.:</strong> {{mobilePhone}}<br />
+	<strong>Email:</strong> <a href="mailto:{{email}}">{{email}}</a><br />
+	<strong>Сайт:</strong> {{{link website website false}}}<br />
+	<strong>ICQ:</strong> <img src="http://status.icq.com/online.gif?icq={{icq}}&img=26" alt="ICQ статус" />{{icq}}
+</address>

Jint.Benchmark/Scripts/template-rendering/content/hello-world/data.json

@@ -0,0 +1,6 @@
+{
+  "lastName": "Pupkin",
+  "firstName": "Vasilii",
+  "patronymic": "Ivanovich",
+  "occupation": "Enikeyschik"
+}

Jint.Benchmark/Scripts/template-rendering/content/hello-world/template.handlebars

@@ -0,0 +1,4 @@
+<div class="greeting">
+	{{!-- Welcome by first name and patronymic --}}
+	Hello, {{firstName}} {{patronymic}}!
+</div>