refactor: replace if/elif unit dispatch with _UNIT_POWERS lookup

[underwoo]

Fixes #4 and #5

Two refactors grouped because they touch the same code.

#4 — Dead initialization

Removed n = self.real — it was always overwritten by the if/elif chain.

#5 — Duplicate _UNIT_POWERS mapping

Replaced the 6-branch if/elif chain with a single dict lookup:

if unit not in _UNIT_POWERS:
    raise AssertionError(f"unhandled unit: {unit!r}")
n = self.real / self._convert ** _UNIT_POWERS[unit]

Adding a new unit is now a single-site change (add to _UNIT_POWERS). Unknown units raise AssertionError instead of silently defaulting to K.

Verification:

• pytest: 47/47 passed
• mypy: clean
• pylint: clean

Closes #4 and #5