Guard clauses — use the BCL throw helpers, not hand-written `if`/`throw`

Use when writing or reviewing argument validation / guard clauses in C#/.NET 6-11 — null checks, empty/whitespace string checks, numeric range and index/bounds checks (ArgumentOutOfRangeException.ThrowIfZero/ThrowIfNegative/ThrowIfGreaterThan/ThrowIfLessThan/etc.), ObjectDisposedException.ThrowIf, replacing hand-written `if (x == null) throw new ...` with the BCL static throw helpers, guarding iterator/async methods eagerly, the CA1510-CA1513/CA2201 analyzer rules, and polyfilling the helpers on netstandard2.0/net4x.