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/0411-protocols-impl-duplicate.sx

@@ -13,6 +13,9 @@
 // both impl modules.
 
 #import "modules/std.sx";
+// `Wrap` is declared in the shared types module; bare-import visibility is
+// non-transitive, so naming it here means importing it here (not via impl-a/b).
+#import "./0411-protocols-impl-duplicate-types.sx";
 #import "./0411-protocols-impl-duplicate-impl-a.sx";
 #import "./0411-protocols-impl-duplicate-impl-b.sx";