wip(E4): partial source-pin + non-transitive flip [stdlib E4 attempt-1 WIP checkpoint]

Incomplete WIP from a worker killed at the 55-min wall (large blast radius:
core source-pin + ~8 example migrations + ~10 library module migrations).
Committed so the resumed session continues on a clean tree. May not build.

library/modules/gpu/gles3.sx

@@ -21,6 +21,7 @@
 //     the Metal backend takes — caller branches on OS).
 
 #import "modules/std.sx";
+#import "modules/allocators.sx";
 #import "modules/compiler.sx";
 #import "modules/opengl.sx";
 #import "modules/gpu/types.sx";

library/modules/gpu/metal.sx

@@ -7,6 +7,7 @@
 // non-iOS targets don't reach the Metal-touching code paths.
 
 #import "modules/std.sx";
+#import "modules/allocators.sx";
 #import "modules/std/objc.sx";
 #import "modules/compiler.sx";
 #import "modules/gpu/types.sx";