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/tuple-baseline.sx

@@ -46,17 +46,17 @@ main :: () -> i32 {
 // ── GAPS (Feature 1 work — intentionally NOT exercised above) ──────
 //
 //   G1. Tuple field projection across elements:
-//         t := (Listenable.{value=1}, Listenable.{value=2});
-//         v := t.value;        // expected: (1, 2)   — Decision 3 "tuple.field"
+//         t := .(Listenable.{value=1}, Listenable.{value=2});
+//         v := t.value;        // expected: .(1, 2)   — Decision 3 "tuple.field"
 //       Today: `error: field 'value' not found on type 'tuple'`.
 //       Needed by canonical `self.sources.value`.
 //
 //   G2. Tuple spread into call args:
-//         p := (10, 20);
+//         p := .(10, 20);
 //         add(..p);            // expected: add(10, 20) — Decision 3 "..tuple"
 //       Today: lowers to one `undef` arg → LLVM arity verification failure.
-//       Needed by canonical `mapper(..sources.value)` and `(..sources)`.
+//       Needed by canonical `mapper(..sources.value)` and `.(..sources)`.
 //
 // Both are already scheduled: parsing in Phase 1.2 (PackExpansion node covers
-// `(..pack)` / `..pack.field`), sema in Phase 2.3 ("tuple-spread parallels").
+// `.(..pack)` / `..pack.field`), sema in Phase 2.3 ("tuple-spread parallels").
 // No separate Feature 1.5 needed — see Step 0.4 triage in CHECKPOINT-LANG.md.