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/1062-errors-value-failable-trailing-expr.sx

@@ -1,4 +1,4 @@
-// Value-carrying failable functions (`-> T !E`) whose body ends in a trailing
+// Value-carrying failable functions (`-> (T, !E)`) whose body ends in a trailing
 // success EXPRESSION (no explicit `return`) must set the success error slot to
 // 0 — the caller's `catch` must NOT fire and the success value must be intact.
 // Regression (issue 0190): `lowerValueBody` used to `coerceToType`+`ret` the
@@ -10,16 +10,16 @@
 E :: error { Bad }
 
 // Single-value trailing-expression success.
-val :: () -> i64 !E { 99 }
+val :: () -> (i64, !E) { 99 }
 
 // String trailing-expression success.
-sval :: () -> string !E { "hi" }
+sval :: () -> (string, !E) { "hi" }
 
 // Multi-value (tuple) trailing-expression success.
-mval :: () -> Tuple(i64, i64) !E { .(1, 2) }
+mval :: () -> (i64, i64, !E) { .(1, 2) }
 
 // A real error still propagates through the value-failable channel.
-fval :: (n: i64) -> i64 !E {
+fval :: (n: i64) -> (i64, !E) {
     if n < 0 { raise error.Bad; }
     n + 1
 }