New Processor and more QoL changes and improvements

Added

• New SOCItemGuidProcessor asset postprocessor that enforces valid and unique GUIDs on collection items at import time, replacing the inline validation in CollectionsAssetsPostProcessor
• Custom CollectionItemQueryPropertyDrawer with foldout UI, rule editing, match type constraint filtering, impossible-rule detection, and human-readable query summaries
• GUID-based lookup caching for collections (CollectionsRegistry) and items (ScriptableObjectCollection) for faster retrieval
• CollectionItemIndirectReference now caches resolved references with a hasCachedRef flag to avoid repeated resolution attempts
• Enhanced error logging in CollectionItemPickerPropertyDrawer with last known names and property paths
• DrawAdditionalIMGUI() extension point in CollectionCustomEditor for subclass customization
• GitHub Copilot review instructions (.github/instructions.md)

Changed

• Renamed CollectionItemQuery.MatchType enum values: NotAny → SomeNot, NotAll → None
• Reduced GC allocations in CollectionItemQuery.Matches() by reusing a HashSet instead of creating a new one per call (by @emtony)
• CollectionItemDropdown now redirects to the collection when the item type has derived types, instead of immediately creating an item
• Code generation now outputs UTF-8 without BOM and handles both \r\n and \n line endings
• Refactored new modifier detection logic in code generation with improved base type filtering
• CollectionItemPicker now uses reverse iteration for safe removal during enumeration
• Refactored CollectionsRegistry.ValidateCollections() to validate items within collections using HashSets for duplicate detection
• Improved bidirectional type assignability check in CollectionsRegistry.GetCollectionsByItemType()
• Robust UXML asset loading in CollectionCustomEditor with error handling for missing visual tree assets

Fixed

• ScriptableObjectCollection<T>.ClearCachedValues() now calls base.ClearCachedValues() to properly clear name/GUID lookup dictionaries
• GUID cache lookups (TryGetItemByGUID, GetCollectionByGUID) now validate the cached entry's GUID still matches before returning, evicting stale entries
• SOCItemGuidProcessor deleted asset handling: entries are now removed by path matching instead of attempting to load already-deleted assets
• Removed UTF-8 BOM from SOCItemGuidProcessor.cs and its .meta file
• Fixed typo IsKnowCollection → IsKnownCollection across multiple files
• Fixed typo collectionLastKnowName → collectionLastKnownName with FormerlySerializedAs for backward compatibility
• Fixed method name SetUsingBaseClassForItems → SetUseBaseClassForItems
• Improved null/destroyed Unity object equality checks in ScriptableObjectCollectionItem
• Removed OnValidate() auto-GUID generation from ScriptableObjectCollectionItem (now handled by the processor)