feat: scan multiple indexes for AND conditions, intersect sorted []Ro…

[RichardKnop]

Scan multiple indexes for AND conditions, intersect sorted []RowID sets in-memory, fetch only surviving rows. Add IntersectResult bool to Scan. setIndexScans marks candidates when intersection cost wins.

[github-actions]

Code Coverage

✅ Total: 70.5% (threshold: 70%)

Package	Coverage
github.com/RichardKnop/minisql	80.7%
github.com/RichardKnop/minisql/e2e_tests	[no
github.com/RichardKnop/minisql/internal/minisql	69.8%
github.com/RichardKnop/minisql/internal/parser	84.2%
github.com/RichardKnop/minisql/pkg/bitwise	100.0%
github.com/RichardKnop/minisql/pkg/lrucache	81.7%