perf(bench): pre-size the HashSet baseline in SparseSetBenchmark Add for a fair comparison

SparseSet's universe is mandatory, so `new SparseSet(universe)` inherently
pre-allocates its sparse array and pays no sparse resize on Add, whereas the
`new HashSet<int>()` baseline paid full incremental-rehash growth — an unfair
per-add comparison. Give the baseline the matching capacity hint
(`new HashSet<int>(ItemCount)`, as the Setup / Remove paths already do) so the
Add category measures per-add cost rather than the baseline's rehash overhead.
Addresses a Copilot review comment on PR #288.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: marius-bughiu

src/Celerity.Benchmarks/SparseSetBenchmark.cs

@@ -46,7 +46,12 @@ public void Setup()
     [BenchmarkCategory("Add")]
     public void HashSet_Add()
     {
-        var set = new HashSet<int>();
+        // Pre-sized to keep the per-add comparison fair: SparseSet's universe is
+        // mandatory, so `new SparseSet(universe)` inherently pre-allocates its sparse
+        // array (no sparse resize on Add). The baseline is given the matching capacity
+        // hint (ItemCount, as the Setup / Remove paths already do) so this measures
+        // per-add cost rather than HashSet's incremental-rehash overhead.
+        var set = new HashSet<int>(ItemCount);
         foreach (var key in keys)
             set.Add(key);
     }