Add optional automatic gaps filling feature for append-only databases

[konard]

Summary

This PR implements the solution for issue #172 by adding an optional automatic gaps filling feature that can be disabled for databases that do not delete data (all changes archived).

Changes Made:

1. Added _enableAutomaticGapsFilling field to UnitedMemoryLinksBase to control runtime existence checks
2. Extended constructor overloads across both UnitedMemoryLinksBase and UnitedMemoryLinks to accept the enableAutomaticGapsFilling parameter
3. Modified the Each method to conditionally skip Exists() checks when gaps filling is disabled
4. Added comprehensive tests to verify both enabled and disabled modes work correctly

Performance Benefits:

For append-only databases (databases that never delete data), disabling automatic gaps filling eliminates runtime Exists() checks, improving performance during link iteration.

API Usage:

// For append-only databases - improved performance
var links = new UnitedMemoryLinks<ulong>(memory, reservationStep, enableAutomaticGapsFilling: false);

// Default behavior (backward compatible)
var links = new UnitedMemoryLinks<ulong>(memory, reservationStep); // enableAutomaticGapsFilling defaults to true

Test Plan:

• ✅ All existing tests pass (22/22, 1 skipped)
• ✅ Added AutomaticGapsFillingDisabledTest to verify skip behavior
• ✅ Added AutomaticGapsFillingEnabledTest to verify default behavior
• ✅ Build succeeds with no breaking changes

This implementation maintains full backward compatibility while providing the requested performance optimization for append-only database scenarios.

🤖 Generated with Claude Code

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Resolves #172