feat: add DESC LIMIT queries to sort_pushdown benchmark

Add q5-q8: ORDER BY l_orderkey DESC LIMIT queries on sorted data.
These test the reverse scan + TopK optimization path (Inexact sort
pushdown) which benefits from RG reorder, stats init, and cumulative
pruning.

q5: DESC LIMIT 100 (narrow projection)
q6: DESC LIMIT 1000 (narrow projection)
q7: DESC LIMIT 100 (wide projection)
q8: DESC LIMIT 1000 (wide projection)

Author: zhuqi-lucas

benchmarks/queries/sort_pushdown/q5.sql

@@ -0,0 +1,7 @@
+-- Reverse scan: ORDER BY DESC LIMIT (narrow projection)
+-- With --sorted: reverse_row_groups=true + TopK + stats init + cumulative prune
+-- Without --sorted: full TopK sort over all data
+SELECT l_orderkey, l_partkey, l_suppkey
+FROM lineitem
+ORDER BY l_orderkey DESC
+LIMIT 100

benchmarks/queries/sort_pushdown/q6.sql

@@ -0,0 +1,5 @@
+-- Reverse scan: ORDER BY DESC LIMIT larger fetch (narrow projection)
+SELECT l_orderkey, l_partkey, l_suppkey
+FROM lineitem
+ORDER BY l_orderkey DESC
+LIMIT 1000

benchmarks/queries/sort_pushdown/q7.sql

@@ -0,0 +1,5 @@
+-- Reverse scan: wide projection + DESC LIMIT
+SELECT *
+FROM lineitem
+ORDER BY l_orderkey DESC
+LIMIT 100

benchmarks/queries/sort_pushdown/q8.sql

@@ -0,0 +1,5 @@
+-- Reverse scan: wide projection + DESC LIMIT larger fetch
+SELECT *
+FROM lineitem
+ORDER BY l_orderkey DESC
+LIMIT 1000