feat: first-class uncertainty

**BEFORE**:
```python
length = 10 ± 0.1 m
time = 2.1 ± 0.2 s
velocity = length / time # Automatically propagates uncertainty
```

**AFTER**:
```python
# After (transpiled.py):
from uncertainties import ufloat
from vyper.units import Quantity, UncertainQuantity

length = UncertainQuantity(ufloat(10, 0.1), 'm')
time = UncertainQuantity(ufloat(2.1, 0.2), 's')
velocity = length / time # Returns UncertainQuantity(4.76 ± 0.47, 'm/s')
```

Start by reading the proposed transpiled.py example and the roles of uncertainties.ufloat, vyper.units.Quantity, and UncertainQuantity. Determine how uncertainty syntax, propagation, and unit-valued results would fit together; done means the shown length, time, and velocity behavior works consistently.

Written by the indexing model from the issue text.