ranges-seam: require a left identifier boundary on the refused spellings #1732

scripts/check-ranges-seam.cmake matches views::stride and ranges::iota with no left identifier boundary. A name such as myviews::stride or foo::views::stride would therefore also be refused. This predates #1728 and fails toward over-matching, which is the safe direction.

Fix: anchor each needle with (^|[^A-Za-z0-9_]). Add a selftest case for a prefixed identifier that must NOT be refused, beside the existing planted violations that must be.