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).

library/modules/std/thread.sx

@@ -106,7 +106,7 @@ Thread :: struct {
 
     // `entry` is the C->sx boundary: abi(.c), fabricates its own
     // Context before touching default-conv sx code (examples/1636).
-    spawn :: (entry: (*void) -> *void abi(.c), arg: *void) -> Thread !ThreadErr {
+    spawn :: (entry: (*void) -> *void abi(.c), arg: *void) -> (Thread, !ThreadErr) {
         t : Thread = .{};
         if pthread_create(@t.handle, null, entry, arg) != 0 { raise error.Spawn; }
         return t;
@@ -144,7 +144,7 @@ Pool :: struct {
 
     // Heap-allocate (the pool must never move: workers hold its address,
     // and it embeds a live mutex), init in place, spawn the workers.
-    create :: (workers: i64, backlog: i64) -> *Pool !ThreadErr {
+    create :: (workers: i64, backlog: i64) -> (*Pool, !ThreadErr) {
         alloc := context.allocator;
         p : *Pool = xx alloc.alloc_bytes(size_of(Pool));
         p.* = Pool.{};