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";