Nethermind integration: replace RocksDB with TrimDB plugin

Add Nethermind as a git submodule and implement TrimDB.Nethermind as a
loadable plugin that replaces RocksDB across all database operations.

- Add Drawaes/nethermind fork as submodule
- Delete local interface copies, use real Nethermind project references
- Implement IDb, ITunableDb, IWriteBatch against real Nethermind interfaces
- Add IColumnsDb<T> support (TrimDbColumnsDb, TrimDbColumnsWriteBatch)
- Add IDbFactory (TrimDbFactory) and INethermindPlugin (TrimDbPlugin)
- Fix TableFileMerger bug: exhausted iterators not removed during initial advance
- Remove TrimDB.Core ReadFlags/WriteFlags (use Nethermind.Core originals)

Author: Drawaes

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "src/TrimDB.Nethermind/nethermind"]
+	path = src/TrimDB.Nethermind/nethermind
+	url = https://github.com/Drawaes/nethermind

src/TrimDB.Core/ReadFlags.cs

@@ -45,9 +45,10 @@ public async ValueTask<bool> MoveNextAsync()
             if (!_hasInitialMove)
             {
                 _hasInitialMove = true;
-                for (var i = 0; i < _activeCount; i++)
+                for (var i = _activeCount - 1; i >= 0; i--)
                 {
-                    await _memoryItems[i]!.MoveNextAsync();
+                    if (!await _memoryItems[i]!.MoveNextAsync())
+                        RemoveIterator(_memoryItems[i]!);
                 }
             }
             else

src/TrimDB.Core/Storage/TableFileMerger.cs

@@ -312,9 +312,6 @@ internal async ValueTask<SearchResult> DoesKeyExistBelowLevel(ReadOnlyMemory<byt
             return SearchResult.NotFound;
         }
 
-        public ValueTask<bool> DeleteAsync(ReadOnlySpan<byte> key, WriteFlags flags)
-            => DeleteAsync(key); // flags ignored for MVP
-
         public ValueTask<bool> DeleteAsync(ReadOnlySpan<byte> key)
         {
             if (Volatile.Read(ref _disposed) == 1) throw new ObjectDisposedException(nameof(TrimDatabase));
@@ -357,9 +354,6 @@ private async ValueTask<bool> DeleteAsyncSlow(byte[] key)
             }
         }
 
-        public Task PutAsync(ReadOnlyMemory<byte> key, ReadOnlyMemory<byte> value, WriteFlags flags)
-            => PutAsync(key, value); // flags ignored for MVP
-
         public async Task PutAsync(ReadOnlyMemory<byte> key, ReadOnlyMemory<byte> value)
         {
             if (Volatile.Read(ref _disposed) == 1) throw new ObjectDisposedException(nameof(TrimDatabase));