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/1302-ffi-objc-block-noop.sx

@@ -9,7 +9,7 @@
 #import "modules/ffi/objc_block.sx";
 
 main :: () -> i32 {
-  cl := () => { print("noop block ran\n"); };
+  cl := () { print("noop block ran\n"); };
   b : Block = xx cl;
   invoke_fn : (*Block) -> void abi(.c) = xx b.invoke;
   invoke_fn(@b);

examples/ffi-objc/1303-ffi-objc-block-capture.sx

@@ -9,7 +9,7 @@
 main :: () -> i32 {
   x : i64 = 42;
   y : i64 = 100;
-  cl := () => { print("x + y = {}\n", x + y); };
+  cl := () { print("x + y = {}\n", x + y); };
   b : Block = xx cl;
   invoke_fn : (*Block) -> void abi(.c) = xx b.invoke;
   invoke_fn(@b);

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;
     };

examples/ffi-objc/1305-ffi-objc-block-inline.sx

@@ -12,7 +12,7 @@ invoke_once :: (b: *Block) {
 
 main :: () -> i32 {
   x : i64 = 7;
-  invoke_once(xx () => {
+  invoke_once(xx () {
     print("inline block, x = {}\n", x);
   });
   0

examples/ffi-objc/1324-ffi-objc-arc-01-autoreleasepool.sx

@@ -34,7 +34,7 @@ main :: () -> i32 {
         // (NSObject.new returns a +1 retained, NOT autoreleased), so this
         // is a smoke test of the helper's shape, not the runtime
         // behavior.
-        autoreleasepool(() => {
+        autoreleasepool(() {
             inner := NSObject.new();
             if inner != null {
                 inner.release();