pLexeme

A single lexeme parser: tries each character/run parser in priority order.

Priority:

1. Newline (handles \r\n normalization)

2. Runnable structural chars (hash, asterisk, underscore, backtick, tilde, hyphen, equals)

3. Runnable whitespace (space)

4. Non-runnable whitespace (tab)

5. Non-runnable structural chars

6. Quote chars

7. Digit runs

8. Text runs (fallback)