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/0051-basic-for-range-bounds.sx

@@ -1,7 +1,9 @@
 // Range bound markers: each side of `..` takes `=` (inclusive) or `<`
 // (exclusive); defaults are start-inclusive, end-exclusive (`a..b` == `a=..<b`).
 // Covers the full matrix, open ranges with start markers, comptime unrolling,
-// runtime bounds, and that `<` / `<<` comparisons still lex normally.
+// runtime bounds, arbitrary expressions at EITHER end (expression parsing
+// stops at the range token), and that `<` / `<<` comparisons still lex
+// normally.
 
 #import "modules/std.sx";
 
@@ -39,6 +41,19 @@ main :: () -> s32 {
     for lo<..=hi (i) { print("{} ", i); }
     print("| lo<..=hi\n");
 
+    // Arbitrary expressions at either end of the range token.
+    x := 2;
+    n := 0;
+    sum := 0;
+    for x+2..=42 (e) { n += 1; sum += e; }      // expression start: 4 .. 42
+    print("x+2..=42: n={} sum={}\n", n, sum);
+    n2 := 0;
+    for x+2<..<x*21 (e) => n2 += 1;             // both ends: 5 .. 41
+    print("x+2<..<x*21: n2={}\n", n2);
+    n3 := 0;
+    for 0..x*3 (i) => n3 += 1;                  // expression end: 0 .. 5
+    print("0..x*3: n3={}\n", n3);
+
     // Comparison operators still lex normally.
     a := 3;
     if a < 5 { print("cmp ok\n"); }

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