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.

src/parser.zig

@@ -2867,9 +2867,12 @@ pub const Parser = struct {
         var capture_by_ref = false;
 
         if (range_end != null) {
-            // Range capture is the optional cursor: `(i)` or nothing.
-            if (self.current.tag == .l_paren) {
+            // Optional cursor, introduced by `:` for symmetry with the
+            // collection form: `for 0..N: (i)` (or `for 0..N` with no cursor).
+            // The colon is required when a cursor is present.
+            if (self.current.tag == .colon) {
                 self.advance();
+                try self.expect(.l_paren);
                 if (self.current.tag != .identifier) return self.fail("expected cursor variable name");
                 capture_name = self.tokenSlice(self.current);
                 self.advance();