test(0051): pin expression bounds at either end of range tokens

x+2..=42 (expression start, 39 iterations summing 897),
x+2<..<x*21 (expressions both ends, 5..41), 0..x*3 (expression end).
Expression parsing stops at the range lexeme from either side, so any
expression works in either position — now pinned.

examples/expected/0051-basic-for-range-bounds.stdout

@@ -8,5 +8,8 @@
 10@5 20@6 30@7 | xs, 5=..
 inline 0<..=3 sum=6
 2 3 4 | lo<..=hi
+x+2..=42: n=39 sum=897
+x+2<..<x*21: n2=37
+0..x*3: n3=6
 cmp ok
 shl=6