Implement blocking script execution order for HTMLScriptElement

[Copilot]

This PR implements document-order script execution for HTMLScriptElement to match classic browser behavior, where scripts execute in the order they appear in the DOM regardless of download completion order.

Problem

Previously, JSAR executed scripts immediately upon compilation completion, which could result in out-of-order execution if later scripts downloaded faster than earlier ones. This behavior differs from standard browser implementations and can break web content that relies on strict script execution order.

Solution

Introduced a script execution queue in BrowsingContext that ensures classic scripts (those without async, defer, or type="module") execute in document order:

<!-- These scripts will now execute in order a → b → c -->
<!-- regardless of which downloads first -->
<script src="script-a.js"></script>
<script src="script-b.js"></script> 
<script src="script-c.js"></script>

<!-- These remain unaffected by the queue -->
<script src="async.js" async></script>        <!-- Executes when ready -->
<script src="defer.js" defer></script>        <!-- Executes after parsing -->
<script type="module" src="module.js"></script> <!-- Module order rules -->

Implementation Details

• Decoupled Architecture: Uses ScriptExecutionHandle with script IDs and weak pointers to avoid circular dependencies between BrowsingContext and HTMLScriptElement
• Queue Management: Added std::deque<ScriptExecutionHandle> to track scripts in document order
• Registration: Classic scripts register with the queue in connectedCallback()
• Execution Control: Scripts wait for their turn instead of executing immediately after compilation
• Minimal Impact: Only affects classic scripts; async, defer, and module scripts retain their existing behavior

Key Features

• ✅ Document Order Preservation: Scripts execute in DOM order regardless of download speed
• ✅ Asynchronous Loading: Network requests remain parallel for performance
• ✅ Selective Application: Only classic scripts use the queue; other script types unchanged
• ✅ Browser Compatibility: Matches standard web browser execution behavior
• ✅ Clean Architecture: Avoids circular dependencies through ID-based handles

Files Changed

• src/client/dom/browsing_context.hpp - Added execution queue interface with decoupled design
• src/client/dom/browsing_context.cpp - Implemented queue management logic using script handles
• src/client/html/html_script_element.hpp - Added queue integration methods
• src/client/html/html_script_element.cpp - Modified execution flow to use queue

This change improves web compatibility by ensuring JSAR's script execution behavior aligns with browser standards while maintaining a clean, decoupled architecture.

Fixes #250.

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