perf!: replace LinkedList with Vector for child node storage

[usernane]

Summary

Replace LinkedList with Vector for HTMLNode child storage, eliminating O(n²) index-access patterns.

Motivation

LinkedList::get($index) is O(n), making loops over children O(n²). With 1000 children, rendering alone took 134ms. Fixes #66.

Changes

• Change $childrenList from LinkedList to Vector in HTMLNode
• Replace removeElement() calls with remove() (Vector API)
• Replace index-based remove() calls with removeAt()
• Add bounds check in getChild() to return null instead of throwing
• Update type hints in private helper methods

How to Test / Verify

All 308 existing tests pass. Benchmark results with 1000 children:

Operation	LinkedList	Vector	Speedup
toHTML(false)	134 ms	15 ms	9x
getChildByID (last)	239 ms	7 ms	35x
index-based loop	105 ms	0.3 ms	363x
addChild() x 1000	25 ms	18 ms	1.4x
Full doc build+render	43 ms	39 ms	1.1x

Breaking Changes and Migration Steps

• children() return type changes from LinkedList to Vector
• Code type-hinting LinkedList for children() return must change to Vector
• Vector::get() throws OutOfRangeException on invalid index (mitigated by bounds check in getChild())

Checklist

• I reviewed my own diff before requesting review
• My commits follow Conventional Commits
• I added/updated tests (or explained why not) — existing 308 tests pass
• I updated docs (if needed) Docs Repo — N/A
• I ran lint/cs-fixer (if applicable) (composer fix-cs) — N/A, minimal change
• I considered backward compatibility
• I considered security

Related issues

Closes #66

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.31%. Comparing base (cf3dfe1) to head (38e0a7a).
⚠️ Report is 15 commits behind head on dev.

Additional details and impacted files

@@             Coverage Diff              @@
##                dev      #79      +/-   ##
============================================
- Coverage     98.44%   98.31%   -0.13%     
- Complexity      898      917      +19     
============================================
  Files             6        6              
  Lines          1799     1843      +44     
============================================
+ Hits           1771     1812      +41     
- Misses           28       31       +3     

Flag	Coverage Δ
php-8.1	?
php-8.2	?
php-8.3	98.31% <100.00%> (-0.02%)	⬇️
php-8.4	?
php-8.5	97.50% <100.00%> (+<0.01%)	⬆️

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