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/0709-modules-issue-0056-diamond-param-impl.sx

@@ -9,6 +9,9 @@
 // registration tripped: `duplicate impl 'Into' for source 's64'`. Now the flat
 // decl list also dedups by node identity, so this builds and prints 7.
 #import "modules/std.sx";
+// `Wrapped` lives in the shared `common.sx`; bare-import visibility is
+// non-transitive, so naming it here means importing it here (not via mid_a/b).
+#import "0709-modules-issue-0056/common.sx";
 #import "0709-modules-issue-0056/mid_a.sx";
 #import "0709-modules-issue-0056/mid_b.sx";