JOURNAL — phase lex

Implementation notes

Used dataclass for Token so equality works naturally in tests.
value field: int for integers, float for floats (including .5 and 10.), str for operators, None for EOF.
Number parsing handles leading-dot (.5) and trailing-dot (10.) cases via a single scan loop tracking has_dot.
LexError extends Exception directly; message includes the char repr and position index.
15 tests cover all DoD items plus edge cases (tab whitespace, $, letter x).