Require complete hexadecimal color values

[OskarEichler]

Summary

Require hexadecimal RGB colors to contain exactly six hexadecimal digits, with the existing optional # prefix.

Reproduction

Current master accepts FF0000tail, a leading newline followed by FF0000, and a trailing newline. The leading-newline case is especially inconsistent: the regexp matches the second line while parsing still starts at byte zero, producing [15, 240, 0].

\A and \z make validation cover the complete input, matching the documented six-digit examples.

Verification

• Existing suite: 220 examples, 0 failures on Ruby 3.2.11 and 4.0.6.
• RuboCop: 15 files, 0 offenses on both Rubies.
• External model: 2,082 checks per Ruby covering case/prefix combinations across 512 RGB values plus malformed prefixes, suffixes, whitespace, newlines, NUL and invalid digits.
• Rebuilt gem: 14 expected files and successful packaged loads on both Rubies.
• No test files changed.

Breaking change

Invalid strings that previously had a valid six-digit prefix are now rejected with ArgumentError. Valid documented inputs are unchanged.