Optimize GetInnerText with style caching and incremental computation

[jafin]

Types of Changes

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• I have read the CONTRIBUTING document
• My code follows the code style of this project

Contribution Type

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• Bug fix (non-breaking change which fixes an issue, please reference the issue id)
• New feature (non-breaking change which adds functionality, make sure to open an associated issue first)
• Breaking change (fix or feature that would cause existing functionality to change)
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• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Description

• Cache computed styles in a dictionary across the entire traversal so the same element is never computed twice (hit as element, parent, and sibling).
• Hoist StyleCollection creation to the top of GetInnerText and reuse it throughout, eliminating repeated stylesheet enumeration and LINQ allocations.
• When the parent's computed style is already cached, use incremental computation (ComputeDeclarationsWithParent) that inherits from the cached parent instead of walking all ancestors - replacing the O(depth) ancestor loop with a single UpdateDeclarations call.

Related #55

Benchmark.net results from similar test to #55

Benchmark	Original	Total Speedup	Mem Original	Mem Now	Mem Reduction
SmallTable (10×5)	10.6 ms	7.7x	13 MB	1.6 MB	88%
LargeTable (200×15)	4,666 ms	10.2x	742 MB	76 MB	90%
LargeTableBody (200×15)	7,558 ms	16.7x	738 MB	76 MB	90%
AllCells individually (3000)	8,039 ms	1.9x	1,309 MB	676 MB	48%

[FlorianRappl]

Definitely a GREAT one. Thanks a lot!