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/generics/0206-generics-generic-into-block.sx

@@ -24,7 +24,7 @@ g_a: i64 = 0;
 g_b: i64 = 0;
 
 main :: () -> i32 {
-    cl := (a: i64, b: i64) => { g_a = a; g_b = b; };
+    cl := (a: i64, b: i64) { g_a = a; g_b = b; };
     blk : Block = xx cl;
 
     invoke_fn : (*Block, i64, i64) -> void abi(.c) = xx blk.invoke;