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.

examples/ffi-objc/1304-ffi-objc-block-multi-arg.sx

@@ -43,7 +43,7 @@ g_sum: i32 = 0;
 g_tag: *void = null;
 
 main :: () -> i32 {
-    cl := (n: i32, tag: *void) => {
+    cl := (n: i32, tag: *void) {
         g_sum = n + 1;
         g_tag = tag;
     };