Replace proportional split with a fill-first loop that allocates
min(remaining, deficit) to each matched month in user-supplied order,
where deficit = expected - already_paid. Prior transactions' contributions
are now properly accounted for, so a second payment on overlapping months
fills only what's still owed instead of splitting proportionally by total
expected. Surplus after all deficits are covered goes to the credit bucket.
Fixes: Matyáš Thér 200+550 showing 566/183 instead of 500/250.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
reconcile() previously split a multi-month payment evenly across months,
which falsely flagged months as underpaid when their expected fees
differed (e.g. 1250 CZK for 02+03+04 2026 with rates 750/350/150 was
shown as 416/month with two months red).
The allocation now runs per matched member: greedy when the share covers
the total expected (each month gets its expected fee, surplus -> credit),
proportional by expected fee otherwise. Out-of-window months keep the
previous even-split-to-credit behavior. 6 new test cases.
Also adds CHANGELOG.md and a changelog convention in CLAUDE.md.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
