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.

readme.md

@@ -498,11 +498,11 @@ operands or to give a value a name distinct from its register. A label that just
 echoes its register (`[rax] "={rax}"`) is rejected.
 
 Outputs decide the result: **0** → `void` (and the asm must be `volatile`);
-**1** → that type; **N** → a tuple, named by each operand's name.
+**1** → that type; **N** → a `Tuple`, named by each operand's name.
 
 ```sx
 // multiple value outputs → a destructurable tuple
-split :: (x: u64) -> (lo: u64, hi: u64) {
+split :: (x: u64) -> Tuple(lo: u64, hi: u64) {
     return asm {
         #string ASM
         and %[l], %[x], #0xff