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/optionals/0923-optionals-narrowing-no-closure-leak.sx

@@ -17,7 +17,7 @@ main :: () {
     if n != null {
         // `n` is narrowed HERE, but the closure body is a separate function:
         // `n` is `?i64` inside it, so this implicit unwrap must be rejected.
-        g := () => { takes_i64(n); };
+        g := () { takes_i64(n); };
         g();
     }
 }

examples/optionals/expected/0923-optionals-narrowing-no-closure-leak.stderr

@@ -1,5 +1,5 @@
 error: cannot use a value of type '?i64' where 'i64' is expected: an optional does not implicitly unwrap; force-unwrap with '!', supply a fallback with '?? <default>', bind it (`if v := ...`), or guard with '!= null'
-  --> examples/optionals/0923-optionals-narrowing-no-closure-leak.sx:20:22
+  --> examples/optionals/0923-optionals-narrowing-no-closure-leak.sx:20:19
    |
-20 |         g := () => { takes_i64(n); };
-   |                      ^^^^^^^^^^^^
+20 |         g := () { takes_i64(n); };
+   |                   ^^^^^^^^^^^^