Refactor LINQ chains to use Count/Sum selector overloads

Simplified LINQ expressions by replacing Where+Count and Select+Sum
chains with the corresponding Count and Sum overloads that accept
a predicate or selector. This reduces intermediate enumerations and
may improve performance.

Author: electricessence

Benchmarks/AllocationBaselineBenchmark.cs

@@ -83,15 +83,13 @@ public int Medium_ForeachOnly()
 	public int Medium_LinqWhere()
 		=> MediumString.AsSegment()
 			.SplitAsSegments(',')
-			.Where(s => s.Length > 5)
-			.Count();
+			.Count(s => s.Length > 5);
 
 	[Benchmark(Description = "Medium - LINQ Select")]
 	public int Medium_LinqSelect()
 		=> MediumString.AsSegment()
 			.SplitAsSegments(',')
-			.Select(s => s.Length)
-			.Sum();
+			.Sum(s => s.Length);
 
 	// ========== Large String Tests ==========
 

Benchmarks/ZLinqImprovementsBenchmark.cs

@@ -190,8 +190,7 @@ public int LinqChain_BCL()
 	public int LinqChain_ZLinq()
 		=> SmallCsv.SplitAsSegments(',')
 			.Where(s => s.Length > 4)
-			.Select(s => s.Length)
-			.Sum();
+			.Sum(s => s.Length);
 
 	// =====================================================================
 	// CATEGORY: ToArray Materialization