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/std/json.sx

@@ -58,6 +58,9 @@
 // =====================================================================
 
 #import "modules/std.sx";
+// `Array`/`Object` methods take an explicit `alloc: Allocator`; bare-import
+// visibility is non-transitive, so the module that names the type imports it.
+#import "modules/allocators.sx";
 #import "modules/fs.sx";
 
 // The writer's failure contract: a too-small caller buffer (Overflow) or