[codex] batch row column fetches in Next

[mattn]

This change reduces cgo traffic in Rows.Next by fetching one row's column metadata and raw values in a single C helper call before converting them on the Go side.

Measured on the same machine with go test -run '^$' -bench 'BenchmarkSuite/(BenchmarkRows|BenchmarkStmtRows)$' -benchmem:

benchmark	before	after	delta
BenchmarkRows	143348 ns/op	135534 ns/op	-5.5%
BenchmarkStmtRows	137675 ns/op	131549 ns/op	-4.5%

Allocations and bytes stayed flat:

benchmark	before allocs	after allocs	before bytes	after bytes
BenchmarkRows	524	524	11128 B/op	11128 B/op
BenchmarkStmtRows	523	523	11104 B/op	11104 B/op

Validated with go test ./... and go test -run '^$' -bench 'BenchmarkSuite/(BenchmarkRows|BenchmarkStmtRows)$' -benchmem.