fix: make min and max aggregates ignore retracted values

[cpruijsen]

groupBy min and max now net each value's multiplicity and only consider values that are still present (multiplicity > 0), the same rule avg / mode and examples/min_max.ts already use. min and max previously ignored multiplicity, so a retracted extreme stayed in the result. Same change in @electric-sql/d2ts and @electric-sql/d2mini. SQLite groupBy re-exports these functions, so it picks up the fix with no extra copy.

Fixes #88

Decision

Consolidate by value, then min/max over net-positive multiplicities. The alternative is to throw when any value has negative net multiplicity, as MultiSet.min / MultiSet.max do. A streaming retract is a normal -1 in this operator, not an illegal collection; throwing would break the path the issue describes. Can switch to the MultiSet semantics if you want that instead.

Test plan

• packages/d2ts tests/operators/groupBy.test.ts, with min and max aggregates: category A is {10, 20, 5}, then +25, then -5 / -25 → minimum: 10, maximum: 20.
• packages/d2mini same test (delta order is retract-then-insert; values must change, not only emit).
• packages/d2ts tests/operators-sqlite/groupBy.test.ts (same assertions; uses the re-exported min / max; needs Node 20 / better-sqlite3 native bindings).
• Optional: two copies of the current max, remove one, max unchanged (net multiplicity stays positive).