lang: multi-iterable for loops — drop ':', add '..=', open ranges, arrow bodies

The for header is now a comma-separated list of iterables with a
positional capture group and no ':' separator:

    for xs (x) { }                    // collection
    for 0..n (i) { }                  // range (end exclusive)
    for 1..=5 (a) { }                 // ..= inclusive end
    for xs, 0.. (x, i) { }            // index idiom (replaces (x, i))
    for xs, ys (x, y) { }             // parallel (zip) iteration
    for xs (x) => sum += x;           // arrow body (full statement)

First-iterable-wins: the first iterable's length drives the loop and
must be bounded; the other positions follow by their own cursors (a
non-first range's end is not consulted or evaluated; a shorter
non-first collection is read past its length on mismatch). The old
single-iterable index capture is replaced by the trailing open range.

Capture/call disambiguation is positional: the paren group immediately
before '{' or '=>' is the capture, every earlier top-level group is a
call. 'for zip(a, b) (x, y)' calls zip; 'for f(n) { }' reads (n) as
the capture and errors with a parenthesize/add-capture hint. The old
':' form errors with a migration hint.

Lowering is unified across forms: one cursor slot per position (ranges
start at their start, collections at 0), all advanced together, the
first position's bound terminating. inline for keeps the single
bounded comptime range.

Migrated the full corpus (examples, library modules, issue repros,
in-source test strings). New coverage: examples/0050 (the full feature
surface) and examples/1149-1155 (seven diagnostic faces). specs.md For
Loop section + grammar rewritten; readme teaser updated.

examples/0022-basic-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 });