lang: require ':' before a for-loop range cursor

The cursor clause now matches the collection form's ': (capture)' — 'for 0..N: (i)' instead of 'for 0..N (i)'. The colon is required when a cursor is present; the no-cursor form 'for 0..N { }' is unchanged. Updated examples/200, the pack-index doc comment, and the spec.

examples/163-pack-runtime-index.sx

@@ -6,7 +6,7 @@
 // constant (a literal, or an `inline for` cursor). A runtime index (here a
 // `while`-loop counter) must produce a clear diagnostic, not the confusing
 // "unresolved 'args'" the slice-index fall-through used to give. To walk a
-// pack, use `inline for 0..args.len (i) { ... }`, which unrolls so each
+// pack, use `inline for 0..args.len: (i) { ... }`, which unrolls so each
 // `args[i]` is a comptime index.
 
 #import "modules/std.sx";

examples/200-for-range.sx

@@ -1,5 +1,5 @@
-// Range-based for loops: `for start..end (i) { }` (cursor optional, `end`
-// exclusive) is a runtime counting loop; `inline for start..end (i) { }`
+// Range-based for loops: `for start..end: (i) { }` (cursor optional, `end`
+// exclusive) is a runtime counting loop; `inline for start..end: (i) { }`
 // is comptime-unrolled — the cursor is a compile-time constant each
 // iteration, so `xs[i]` over a heterogeneous pack substitutes the concrete
 // per-position element (this is what drives pack iteration).
@@ -16,14 +16,14 @@ impl Show for B { show :: (self: *B) -> string => "B"; }
 
 // Comptime-unrolled iteration over a pack; cursor `i` indexes the pack.
 each :: (..xs: Show) -> void {
-    inline for 0..xs.len (i) {
+    inline for 0..xs.len: (i) {
         print("[{}]={}\n", i, xs[i].show());
     }
 }
 
 main :: () -> s32 {
     // Runtime range, cursor used.
-    for 0..3 (i) { print("i={}\n", i); }
+    for 0..3: (i) { print("i={}\n", i); }
 
     // Runtime range, no cursor — body runs `end - start` times.
     n := 0;
@@ -31,7 +31,7 @@ main :: () -> s32 {
     print("n={}\n", n);
 
     // Non-zero start.
-    for 2..5 (j) { print("j={}\n", j); }
+    for 2..5: (j) { print("j={}\n", j); }
 
     // Inline unroll over a heterogeneous pack.
     each(A.{ x = 1 }, B.{ s = "hi" }, A.{ x = 3 });