## DONE

Phase: lex — tokenizer

All DoD items self-certified (BUILD phase — deferred Adversary review).

### D1 — numbers
WHAT: integers and floats tokenize to NUMBER with int/float value; EOF appended.
HOW: `python -c "from calc.lexer import tokenize; print([(t.kind,t.value) for t in tokenize('42')])"`
EXPECTED: `[('NUMBER', 42), ('EOF', None)]`
WHERE: calc/lexer.py

### D2 — operators & parens
WHAT: `+ - * / ( )` each produce correct kind token.
HOW: `python -c "from calc.lexer import tokenize; print([t.kind for t in tokenize('1+2*3')])"`
EXPECTED: `['NUMBER', 'PLUS', 'NUMBER', 'STAR', 'NUMBER', 'EOF']`
WHERE: calc/lexer.py

### D3 — whitespace & errors
WHAT: spaces/tabs skipped; invalid char raises LexError with char + position.
HOW: `python -c "from calc.lexer import tokenize; tokenize('1 @ 2')"`
EXPECTED: raises `calc.lexer.LexError: unexpected character '@' at position 2`
WHERE: calc/lexer.py

### D4 — tests green
WHAT: 14 unittest tests in calc/test_lexer.py, 0 failures.
HOW: `python -m unittest -q`
EXPECTED: `Ran 14 tests in 0.000s\nOK`
WHERE: calc/test_lexer.py

### Plan verify commands (verbatim from lex.md)
```
python -m unittest -q
python -c "from calc.lexer import tokenize; print([(t.kind,t.value) for t in tokenize('3.5*(1-2)')])"
python -c "from calc.lexer import tokenize; tokenize('1 @ 2')"
```
EXPECTED outputs:
1. `Ran 14 tests in 0.000s\nOK`
2. `[('NUMBER', 3.5), ('STAR', '*'), ('LPAREN', '('), ('NUMBER', 1), ('MINUS', '-'), ('NUMBER', 2), ('RPAREN', ')'), ('EOF', None)]`
3. `calc.lexer.LexError: unexpected character '@' at position 2`