⚡ Bolt: Cache RegExp instances in NexusPHP adapters to improve parsing performance

[JustLookAtNow]

💡 What:
Extracted dynamically compiled RegExp objects into cached variables:

1. NexusPHPWebAdapter: Created top-level _sizeRegExp and _relativeUrlRegExp.
2. NexusPHPAdapter: Created static final _whitespaceRegExp.

🎯 Why:
Compiling RegExp dynamically within loops and build methods in Dart is a known performance bottleneck, particularly for isolate functions processing multiple items (like search parsing) and loops over collections.

📊 Impact:
Prevents repeated regex compilations during HTML/API parsing and mapping processes. This provides a minor but measurable CPU improvement when evaluating hundreds of torrent items or mapping extensive tracker categories.

🔬 Measurement:
Run flutter test specifically for adapter parsing logic (flutter test test/nexusphp_adapter_test.dart test/nexusphp_web_adapter_test.dart) to ensure the correct behavior is maintained post-optimization. No visual changes; entirely CPU optimization on parsing logic.

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PR created automatically by Jules for task 12816164061257615172 started by @JustLookAtNow

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