perf(array): add dense fast path for Array.prototype.splice

[tkshsbcue]

Related to #3407.

Right now splice on a dense array still goes through the full generic path — individual Get/Set/DeletePropertyOrThrow per element. Same approach as the existing shift fast path, this skips all of that and operates on the ThinVec directly via ThinVec::splice.

Handles DenseI32, DenseF64, and DenseElement. If replacement items don't match the storage type (e.g. string into an i32 array), converts to DenseElement first.

The fast path only kicks in when:

• It's a real array with the default shape (no subclass / custom @@species)
• itemCount <= actualDeleteCount so no new indices are created beyond len - 1, avoiding the prototype-chain-setter issue from Optimize array unshift #5076
• len <= vec.len() (storage matches length)

Falls through to the generic path otherwise.

~25% faster on a benchmark doing repeated splices on 500-element dense arrays.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	53,125	53,125	0
Passed	51,049	51,049	0
Ignored	1,482	1,482	0
Failed	594	594	0
Panics	0	0	0
Conformance	96.09%	96.09%	0.00%

Tested main commit: 3cec5e6bc4454077cb8d65b537d368d633137e0e
Tested PR commit: 1536cdec1b4630e616af61c0d928e31970fec8c1
Compare commits: 3cec5e6...1536cde

[codecov]

Codecov Report

❌ Patch coverage is 56.41026% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.79%. Comparing base (6ddc2b4) to head (1536cde).
⚠️ Report is 951 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/builtins/array/mod.rs	56.41%	17 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5328       +/-   ##
===========================================
+ Coverage   47.24%   59.79%   +12.55%     
===========================================
  Files         476      590      +114     
  Lines       46892    63734    +16842     
===========================================
+ Hits        22154    38111    +15957     
- Misses      24738    25623      +885     

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