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/event.sx

@@ -116,7 +116,7 @@ Loop :: struct {
     // long-lived-container rule).
     own: Allocator;
 
-    init :: () -> Loop !EventErr {
+    init :: () -> (Loop, !EventErr) {
         e := ep.ep_create();
         if e < 0 { raise error.Init; }
         return Loop.{ epfd = e, regs = .{}, own = context.allocator };
@@ -216,7 +216,7 @@ Loop :: struct {
 
     // Fill `out` with ready events, waiting at most `timeout_ms`
     // (negative = forever). Returns the count; 0 is a timeout.
-    wait :: (self: *Loop, out: []Event, timeout_ms: i64) -> i64 !EventErr {
+    wait :: (self: *Loop, out: []Event, timeout_ms: i64) -> (i64, !EventErr) {
         raw : [64]ep.EpollEvent = ---;
         cap : i64 = 64;
         if xx out.len < cap { cap = xx out.len; }
@@ -254,7 +254,7 @@ Loop :: struct {
 Loop :: struct {
     kq: i32 = -1;
 
-    init :: () -> Loop !EventErr {
+    init :: () -> (Loop, !EventErr) {
         q := kqb.kqueue();
         if q < 0 { raise error.Init; }
         return Loop.{ kq = q };
@@ -298,7 +298,7 @@ Loop :: struct {
 
     // Fill `out` with ready events, waiting at most `timeout_ms`
     // (negative = forever). Returns the count; 0 is a timeout.
-    wait :: (self: *Loop, out: []Event, timeout_ms: i64) -> i64 !EventErr {
+    wait :: (self: *Loop, out: []Event, timeout_ms: i64) -> (i64, !EventErr) {
         raw : [64]kqb.Kevent = ---;
         cap : i64 = 64;
         if xx out.len < cap { cap = xx out.len; }