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.

current/CHECKPOINT-LANG.md

@@ -0,0 +1,51 @@
+# CHECKPOINT-LANG — user-facing language features
+
+Companion to [PLAN-LANG.md](PLAN-LANG.md). Update after every step (one step at
+a time, per the cadence rule).
+
+## Last completed step
+**Tuple syntax cutover — `Tuple(...)` type + `.(...)` value (commit 989e18b7).**
+The bare-paren tuple grammar was replaced with explicit, position-unambiguous
+forms that mirror how structs work:
+
+- type     `(A, B)`         → `Tuple(A, B)`         (named keeps `:` — `Tuple(x: A, y: B)`)
+- value    `(a, b)`         → `.(a, b)`             (named uses `=` — `.(x = a, y = b)`)
+- typed    (new)            → `Tuple(A, B).(a, b)`  (like `Point.{...}`)
+- failable `-> (T, !)`      → `-> T !`
+           `-> (T1, T2, !)` → `-> Tuple(T1, T2) !`  (error channel OUTSIDE the Tuple)
+
+Bare `(...)` is now grouping ONLY, everywhere; a comma in bare parens is a hard
+error with a migration hint. Grouping, function types `(A, B) -> R`, param lists,
+lambdas, match bindings, and `?(?T)` grouping are unaffected. `Tuple(...)` is
+strictly a TYPE in every position (incl. `size_of` / `type_info` args); a tuple
+VALUE comes only from `.(...)` or `Tuple(...).(...)`. A bare `Tuple(1, 2)`
+(non-type elements) is rejected. Field access is unchanged (`.0`/`.1` positional,
+`.x` named). Optional semantics are untouched — `??T ≡ ?T` was NOT done; nested
+optionals (`?(?i64)`) stay genuine.
+
+The ~110 tuple-bearing corpus files were migrated by a one-shot AST-aware
+migrator; new examples landed (0130 new syntax, 0131 typed construction, 1060
+named-tuple failable return). Issue **0189** filed (non-type expression in type
+position silently fabricates an empty struct — surfaced while validating the
+`Tuple(i32, g.a)` rejection path).
+
+Docs updated to the new syntax: `specs.md` (Tuple Types section, function
+multi-return note, all error-channel sections, Variadic Heterogeneous Type Packs,
+Tuple UFCS Splatting, and the normative Grammar block) and `readme.md` (inline-asm
+named-tuple return + the `N → a tuple` rule). Stale old-syntax mentions in example
+header comments were corrected (comments only — no code touched). Suite green
+(810 ran, 0 failed).
+
+## Current state
+Tuple syntax cutover shipped and documented. `Tuple(...)` / `.(...)` are the only
+tuple spellings across the corpus, specs, and readme.
+
+## Next step
+Pick up the next incomplete LANG step from [PLAN-LANG.md](PLAN-LANG.md).
+
+## Log
+- **Tuple syntax cutover** (commit 989e18b7): `(A,B)`/`(a,b)` tuples replaced by
+  `Tuple(A,B)` type + `.(a,b)` value; failable `!` moved outside the Tuple
+  (`-> T !` / `-> Tuple(...) !`); bare parens are grouping-only. Docs (specs.md +
+  readme.md) and stale example-comment mentions migrated to the new syntax. Issue
+  0189 filed. Suite green (810 ran, 0 failed).