perf(table): reuse RowScanner across Iter rows

Iter created a fresh scany RowScanner per row via pgxscan.API.ScanRow,
recomputing the column-to-field mapping on every iteration. Create one
RowScanner before the loop and reuse it so the reflection work happens
once per query instead of once per row.

Benchmarked via BenchmarkTableIter (in-memory fake pgx.Rows), count=8:

  rows=1000   time  394.8µs -> 177.5µs  (-55%)
              B/op  406.6Ki ->  63.2Ki  (-84%)
              allocs   6002 ->    1008  (-83%, ~6/row -> ~1/row)

All deltas p=0.000. GetAll/List/pagination already reuse a single
RowScanner internally, so this only affects the Iter streaming path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: VojtechVitek

table.go

@@ -565,12 +565,18 @@ func (t *Table[T, P, I]) Iter(ctx context.Context, where sq.Sqlizer, orderBy []s
 }
 
 // iterRows yields records scanned from rows, closing rows when iteration ends.
+//
+// A single RowScanner is created once and reused across every row: it caches
+// the column-to-field mapping after the first Scan, so the reflection work is
+// done once per query rather than once per row (which pgxscan.API.ScanRow
+// would do by allocating a fresh RowScanner each call).
 func (t *Table[T, P, I]) iterRows(rows pgx.Rows) iter.Seq2[P, error] {
 	return func(yield func(P, error) bool) {
 		defer rows.Close()
+		rs := t.Query.Scan.NewRowScanner(rows)
 		for rows.Next() {
 			var record T
-			if err := t.Query.Scan.ScanRow(&record, rows); err != nil {
+			if err := rs.Scan(&record); err != nil {
 				yield(nil, err)
 				return
 			}