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/0719-modules-cli-and-json.sx

@@ -12,6 +12,7 @@
 // independent identities.
 
 #import "modules/std.sx";
+#import "modules/allocators.sx"; // `Allocator` is non-transitive: name it, import it.
 // `cli` is imported BOTH flat (so its types — `FlagSpec` / `Command` / `Diag` —
 // are bare-visible) AND namespaced (so the same-name `cli.parse` stays a
 // distinct qualified identity from `json.parse`). Post-E1 a bare reference to a