for arr: (it) {}

specs.md

@@ -789,22 +789,23 @@ while i < 10 {
 
 ### For Loop
 ```sx
-for iterable {
-  // `it` is the current element
-  // `it_index` is the current index (s64)
-  print("{it}\n");
-}
+for iterable: (elem) { }            // element alias (no copy)
+for iterable: (elem, ix) { }        // element + index
+for iterable: (_, ix) { }           // index only
 ```
-Iterates over arrays and slices. The loop body has two implicit variables:
-- `it` — the current element value
-- `it_index` — the current index (s64, starting at 0)
+Iterates over arrays and slices. The capture clause after `:` binds loop variables:
+- The first name is the element capture (non-reassignable alias into the array/slice)
+- The optional second name is the index (s64, starting at 0, also non-reassignable)
+- Use `_` to discard a capture
+
+The element capture is a direct alias — reads and field writes go to the original array element. Direct reassignment of the capture (`elem = x`) is a compile error.
 
 `break;` exits the loop. `continue;` skips to the next iteration.
 ```sx
 arr : [5]s32 = .[1, 2, 3, 4, 5];
-for arr {
-    if it_index == 2 { continue; }
-    print("{it}\n");
+for arr: (val, ix) {
+    if ix == 2 { continue; }
+    print("{}\n", val);
 }
 ```
 
@@ -1069,7 +1070,7 @@ assignment      = lvalue ('=' | '+=' | '-=' | '*=' | '/=') expr
 lvalue          = IDENT | postfix '.' IDENT
 expr            = if_expr | match_expr | while_expr | for_expr | lambda | binary
 while_expr      = 'while' expr block
-for_expr        = 'for' expr block
+for_expr        = 'for' expr ':' '(' IDENT [',' IDENT] ')' block
 binary          = unary (binop unary)*
 unary           = ('-' | '!' | 'xx' | 'cast' '(' type ')') postfix
                 | postfix