Document CPU sieve invariants and cache boolean guards

PerfectNumbers.Core.Tests/PrimeTester/PrimeTesterIsPrimeTests.cs

@@ -8,6 +8,8 @@ public class PrimeTesterIsPrimeTests
 {
     [Theory]
     [Trait("Category", "Fast")]
+    // EvenPerfectBitScanner never routes these small values, but factoring helpers and residue checks still call into
+    // PrimeTester for sanity probes, so the regression suite keeps the tiny cases alongside large primes/composites.
     [InlineData(1UL, false)]
     [InlineData(2UL, true)]
     [InlineData(3UL, true)]