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.

examples/0713-modules-json-writer.sx

@@ -18,6 +18,7 @@
 // and freed in one `deinit`; the writer path allocates nothing.
 
 #import "modules/std.sx";
+#import "modules/allocators.sx"; // `Allocator` is non-transitive: name it, import it.
 #import "modules/std/json.sx";
 #import "modules/fs.sx";