perf: specialize ArrowFunction object to bypass generic call overhead

[MayankRaj435]

This PR introduces a dedicated ArrowFunction struct that implements a specialized __call__ handler, bypassing the constructor check and this-binding resolution that are unnecessary for arrow functions.

Motivation: This directly addresses the discussion in #5264 about optimizing function calls at the object layer rather than the VM/opcode layer, as suggested by @jedel1043.

Changes

core/engine/src/builtins/function/mod.rs

• New ArrowFunction struct: A lightweight alternative to OrdinaryFunction that stores only the fields relevant to arrow functions (code, environments, home object, realm, script/module).
• Specialized arrow_function_call handler: Skips the is_class_constructor check and this-binding resolution. Arrow functions are never constructors and always use lexical this, so these checks are pure overhead.

core/engine/src/vm/code_block.rs

• Updated create_function_object and create_function_object_fast to detect arrow functions (code.is_arrow()) and instantiate the specialized ArrowFunction instead of the generic OrdinaryFunction.

Benchmarks

Microbenchmark: 100 million arrow function calls in release mode.

Metric	Baseline (OrdinaryFunction)	Optimized (ArrowFunction)	Improvement
Execution Time	21.0s	15.6s	~26% Speedup

Design Rationale

Per the discussion in #5264, I measured size_of::<Instruction>() at 24 bytes. Adding new opcodes to the VM dispatch table risks increasing the Instruction enum size and causing D-cache pressure. This object-layer approach achieves comparable performance gains while keeping the VM instruction set clean.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,732	50,732	0
Ignored	1,426	1,426	0
Failed	805	805	0
Panics	0	0	0
Conformance	95.79%	95.79%	0.00%

Tested main commit: f075094f9674f9919b4d2e85ca1bcba410bf34b0
Tested PR commit: 9dd693a764f5a1eae89b287608f6ef1338ed0354
Compare commits: f075094...9dd693a

[MayankRaj435]

@jedel1043 sir please review it at your convenience ,thanks

[codecov]

Codecov Report

❌ Patch coverage is 69.82249% with 51 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.69%. Comparing base (6ddc2b4) to head (9dd693a).
⚠️ Report is 921 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/builtins/function/mod.rs	65.65%	34 Missing ⚠️
core/engine/src/vm/code_block.rs	84.61%	6 Missing ⚠️
core/engine/src/vm/opcode/function.rs	64.28%	5 Missing ⚠️
core/engine/src/builtins/eval/mod.rs	66.66%	3 Missing ⚠️
...e/src/environments/runtime/declarative/function.rs	50.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5273       +/-   ##
===========================================
+ Coverage   47.24%   59.69%   +12.45%     
===========================================
  Files         476      589      +113     
  Lines       46892    63595    +16703     
===========================================
+ Hits        22154    37966    +15812     
- Misses      24738    25629      +891     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.