Fix SCSS single-line comments in selector state - #3336

SCSS declaration content can remain in the selector state after a property value. That state did not recognize // comments, so comments following declarations were highlighted as operators and selector names.

Add the comment rule to the SCSS selector state and protect URL contents before it runs. Quoted, unquoted, escaped and interpolated URLs keep their slashes as string content, including leading newlines and declarations without a space after :. Shared Sass, CSS and LESS rules remain unchanged.

Refs #2852. The LESS portion was addressed by #3005; this change covers the remaining SCSS selector/declaration path. It does not claim to fix the indented Sass portion. #3265 addresses percentages and placeholder selectors in the shared rules and covers a separate defect.

Validation on CPython 3.13.0:

• 36 focused semantic cases pass, with retained failures on the unchanged code and intermediate URL guards.
• Full suite: 5,367 passed, 16 skipped. Skips cover three existing guess cases, four SRCINFO filename cases, eight optional PIL formatter cases and the absent LaTeX executable.
• All five configured source checks pass, including Ruff 0.15.2. These were direct executions of the configured commands; a complete tox environment/platform matrix was not run.
• The new SCSS golden was generated with the repository's --update-goldens path, reviewed and then checked normally. The real CLI, 15 bounded malformed-input controls and unchanged neighboring lexer token streams were also checked. External nsgmls/LaTeX validation, docs, regexlint and a coverage report were not available or run.

OpenAI Codex assisted the investigation, implementation, tests and description. A separate Codex agent reviewed the final source and retained verification evidence.