refactor: canonical failable syntax (T, !) — remove the bare -> T ! sugar

The trailing-`!`-after-the-value-type spelling (`-> T !`, `-> Tuple(A,B) !`) was a
redundant second way to write a failable return that the parser folded into the
same AST as the parenthesized `(T, !)` / `(A, B, !)` result list. Remove it so
there is ONE canonical spelling: the error channel always rides as the last slot
of the parenthesized list.

- parser: `parseFnReturnType` no longer folds a trailing `!` after a value type —
  it rejects it with a located diagnostic ("a failable return is written `(T, !)`
  … not `T !`"). This one chokepoint covers fn declarations, lambdas, fn-pointer
  types `(A) -> R`, and closure types `Closure(A) -> R`. The error-ONLY `-> !` /
  `-> !ErrSet` form is unaffected (parsed by parseTypeExpr as an error_type_expr).
- migrated every usage to canonical form across library/ + examples/ + issues/ +
  tests/: `-> T !E` → `-> (T, !E)`; the value-carrying `-> Tuple(A, B) !` (which
  FLATTENED to a multi-value failable) → `-> (A, B, !)`, preserving behavior. A
  genuine single-tuple-value failable stays `-> (Tuple(A,B), !)`.
- parser unit tests: the "bare form folds" tests become "bare form is rejected";
  canonical-form parse tests retained.
- docs: specs.md §12 + scattered refs and readme.md updated to the `(T, !)` form.

Behavior-preserving (the bare form was sugar for the same AST). Adversarial review
confirmed: rejection complete across all positions, every canonical form works on
both success/error paths, error-only `-> !` intact, no crashes. Full suite green
(unit tests + 850 corpus examples).

examples/errors/1041-errors-failable-closure-shape-union.sx

@@ -10,7 +10,7 @@
 All :: error { Negative, Other }
 
 // `h` is a bare-`!` Closure slot; the caller declares the union as `!All`.
-dispatch :: (h: Closure(i32) -> i32 !, x: i32) -> i32 !All {
+dispatch :: (h: Closure(i32) -> (i32, !), x: i32) -> (i32, !All) {
     return try h(x);
 }
 
@@ -19,9 +19,9 @@ main :: () -> i32 {
     push Context.{ allocator = xx gpa } {
         // Two literals of the SAME shape raising DIFFERENT tags both feed the
         // one shared `Closure(i32)->(i32,!)` union node.
-        handlers : List(Closure(i32) -> i32 !) = .{};
-        handlers.append(closure((x: i32) -> i32 ! { if x < 0 { raise error.Negative; } return x * 2; }));
-        handlers.append(closure((x: i32) -> i32 ! { if x == 0 { raise error.Other; } return x + 100; }));
+        handlers : List(Closure(i32) -> (i32, !)) = .{};
+        handlers.append(closure((x: i32) -> (i32, !) { if x < 0 { raise error.Negative; } return x * 2; }));
+        handlers.append(closure((x: i32) -> (i32, !) { if x == 0 { raise error.Other; } return x + 100; }));
 
         // success paths
         print("ok0={}\n", dispatch(handlers.items[0], 5) catch (e) 0);   // 10