Make invoice maths exact: half-up line rounding, per-rate VAT, largest-remainder allocation, banker's FX rounding

# Task: make the invoice maths exact `money.py` uses binary floats and produces off-by-a-cent results. Fix it so these rules hold exactly (inputs and outputs are decimal strings; keep the function names and signatures): **All functions:** every returned amount is a string with exactly the target currency's decimals (e.g. "0.00", "-1.50", "1200" for JPY, "1.001" for BHD). Never output a negative zero such as "-0.00": a zero is always unsigned. `currency`/`to_currency` default to EUR where shown. **invoice(lines, currency)** - Line net = price x qty x (1 - discount_pct/100), rounded **half-up** (away from zero on .5) to the currency's decimals (EUR/USD 2, JPY 0, BHD 3). `qty` may be fractional, `discount_pct` is optional (default 0). Negative prices (credit lines) are allowed: half-up means 0.5 rounds away from zero for them too. - `net` = sum of rounded line nets. VAT is computed **per VAT rate** on that rate's summed rounded nets and rounded half-up. `vat` maps each rate string exactly as given (e.g. "20", "5.5") to its amount, in first-appearance order. `gross` = net + sum of VAT amounts. **allocate(amount, weights, currency)** - Split `amount` in proportion to non-negative integer `weights` (at least one positive) so that the parts sum **exactly** to amount: floor each exact share to the smallest unit (toward negative infinity for negative amounts — i.e. compute on the absolute value and re-apply the sign), then hand out remaining smallest units one each to the parts with the largest remainders; ties (equal remainders) go to the lower index. Example: `allocate("0.05", [3, 7])` -> `["0.02", "0.03"]` (exact shares 1.5 and 3.5 cents, equal remainders, the extra cent goes to index 0). Zero weight gets zero. Raise `ValueError` if all weights are 0. **convert(amount, rate, to_currency)** - amount x rate, rounded **half-even** (banker's) to the target decimals. Standard library only. No network.