DECISIONS (append-only, shared)

lex phase

Token as dataclass: Used @dataclass for Token so future parser phases get equality, repr, and field access for free.
Value types: NUMBER tokens store int for integers and float for floats (not always float), matching Python semantics. Parser/evaluator can use isinstance to distinguish.
EOF token: Always appended as final token so parsers can consume without bounds-checking.
LexError message format: "unexpected character {ch!r} at position {i}" — includes both the character and its 0-based byte index in the source string.