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/closures/0304-closures-enum-arg-through-closure-field.sx

@@ -10,7 +10,7 @@ Ctx :: struct {
 }
 
 main :: () -> i32 {
-  c : Ctx = .{ on = (f: Fmt) => {
+  c : Ctx = .{ on = (f: Fmt) {
     n : i64 = xx f;
     print("cl f = {}\n", n);
   }};