style: migrate arrow-block lambdas () => { .. } to () { .. }

The canonical sx block-body lambda is `(params) { stmts }` (and
`(params) -> Ret { stmts }`); the arrow form `=>` is for EXPRESSION bodies
(`(params) => expr`). The arrow-block hybrid `(params) => { .. }` was being
used in 33 files — convert all of them by dropping the `=>`. The two forms are
exactly equivalent (verified: identical IR and identical runtime values — the
block tail is the value with or without a `-> Ret`), so this is a pure source
cleanup: no `.ir` churn, and the only snapshot change is 0923's diagnostic
COLUMN (a negative narrowing test whose error span shifted by the removed `=> `).

Arrow EXPRESSION bodies (`=> expr`, `=> .{..}`, `=> [..]`) and `=>` inside
comments/strings were left untouched. Migrated across examples/concurrency,
examples/{closures,ffi-objc,generics,optionals,types}, issues/, and the stdlib
(io.sx, sched.sx). Suite 855/0.

issues/0156-comptime-pack-captured-into-closure.sx

@@ -14,7 +14,7 @@ main :: () {
     w := (a: i64, b: i64) -> i64 => a + b;
     t := .{40, 2};
     out : i64 = 0; po := @out;
-    captured :: () => { po.* = w(..t); };   // tuple spread inside a closure → panics
+    captured :: () { po.* = w(..t); };   // tuple spread inside a closure → panics
     captured();
     print("out: {}\n", out);                // want: out: 42 (or a clean diagnostic)
 }