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/probes/pack-expansion-parses.sx

@@ -6,12 +6,12 @@
 // arrive in Phase 2 — do NOT expect this to compile/run yet. The authoritative
 // checks are the parser unit tests in src/parser.zig ("parse pack expansion: …").
 
-// 1. Tuple value position — `(..pack)` / `(..pack.field)`:
+// 1. Tuple value position — `.(..pack)` / `.(..pack.field)`:
 tv1 :: () => .(..xs);
 tv2 :: () => .(..xs.value);
 tv3 :: () => .(a, ..xs, b);            // mixed positional + spread
 
-// 2. Tuple type position — `(..F(Ts))` / `(..F(Ts.Arg))`:
+// 2. Tuple type position — `Tuple(..F(Ts))` / `Tuple(..F(Ts.Arg))`:
 tt1 :: (x: Tuple(..ValueListenable(Ts)))       => x;
 tt2 :: (x: Tuple(..ValueListenable(Ts.Arg)))   => x;