Real catalog has 3 different VAT rates (5%, 8%, 10%) - the algorithm
previously mixed products across rates freely within one combo. Each
attempt now picks a random pivot product to fix that attempt's tax rate,
then only combines within that same-rate subset. Pivot selection is
weighted naturally by group size (uniform draw over all candidates), so
larger rate groups are proportionally more likely to be tried first.

Verified against the real catalog: all previously-working target amounts
still succeed, every returned combo uses exactly one VAT rate.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017KPzWwuTEeX2vXGXvyGn4q