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/0808-memory-xx-value-routes-through-context-allocator.sx

@@ -9,6 +9,7 @@
 // the operand's storage, so it never allocates and never reaches this
 // path. See specs.md §3 — Protocol value ownership and lifetime.
 #import "modules/std.sx";
+#import "modules/allocators.sx"; // `Allocator` is non-transitive: name it, import it.
 
 Tracer :: struct {
     count: s64;