Add readonly/const models for collection concepts

[konard]

Summary

Resolves issue #78 by introducing readonly models for each collection concept that preserve const qualifiers instead of stripping them with std::remove_const_t.

• CReadonlyDictionary: Readonly dictionary concept with only const methods (find, contains, empty, size)
• CReadonlyList: Readonly list concept with only const methods (size, empty, indexing)
• CReadonlySet: Readonly set concept with only const methods (find, contains, size, empty)

Key Changes

1. CDictionary.h: Added CReadonlyDictionaryHelpFunction that uses const Self instead of std::remove_const_t<TRawSelf>
2. CList.h: Added CReadonlyListHelpFunction with const-only operations
3. CSet.h: Added CReadonlySetHelpFunction with const-only operations
4. Test example: Demonstrates that const-qualified types now work properly with readonly concepts

Problem Solved

The original issue was that all collection concepts used std::remove_const_t<TRawSelf> which stripped away const qualifiers, making it impossible to define readonly models that only use const methods. This was described as a "small hack" that excluded readonly collections.

Solution

Instead of always removing the const qualifier, the new readonly concepts preserve it by using TRawSelf directly, ensuring that only const methods are required and available.

Test Plan

• Compilation test passes with g++ -std=c++20
• Static assertions verify const-qualified types work with readonly concepts
• Static assertions verify non-const types still work with original concepts
• All existing functionality preserved

🤖 Generated with Claude Code

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Resolves #78