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/comptime/0623-comptime-metatype-tuple.sx

@@ -3,7 +3,7 @@
 // tuple here). Tuples are POSITIONAL, so `TupleInfo` is just a `[]Type` (no field
 // names). Two paths:
 //   1. Programmatic build: `define(declare("Pair"), .tuple(.{ elements = … }))`.
-//   2. Round-trip: `define(declare("TripleCopy"), type_info((i64, bool, f64)))`
+//   2. Round-trip: `define(declare("TripleCopy"), type_info(Tuple(i64, bool, f64)))`
 //      reflects a source tuple type INTO a `.tuple(TupleInfo)` value and
 //      reconstructs it — no literal element list.
 #import "modules/std.sx";