docs: tuple syntax cutover — Tuple(...) type, .(...) value, channel-outside-Tuple failables

Rewrite specs.md tuple/failable/pack/UFCS/grammar sections to the new
syntax, update readme.md, and refresh stale tuple references in example
header comments. Also fixes two pre-existing doc inaccuracies surfaced in
review: drop the value-discarding `;` in the tuple-return examples, and
correct the §13 function-type grammar production (optional param list +
optional trailing `!` channel). Optional semantics unchanged.

current/CHECKPOINT-LANG.md logs the cutover.

examples/basic/0036-basic-ufcs-aliases.sx

@@ -35,7 +35,7 @@ main :: () {
         print("{}\n", a);                // 2
         print("{}\n", b);                // 1
 
-        wrap :: (x: i64) -> Tuple(i64) { .(x) }   // 1-tuple needs trailing comma; (i64) groups
+        wrap :: (x: i64) -> Tuple(i64) { .(x) }   // 1-tuple type `Tuple(i64)`; bare `(i64)` groups
         t := wrap(99);
         print("{}\n", t.0);             // 99
     }