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/expected/0411-protocols-impl-duplicate.stderr

@@ -1,5 +1,5 @@
 error: duplicate xx conversion from 's64' to 'Wrap': impls in /Users/agra/projects/sx/examples/./0411-protocols-impl-duplicate-impl-a.sx and /Users/agra/projects/sx/examples/./0411-protocols-impl-duplicate-impl-b.sx
-  --> /Users/agra/projects/sx/examples/0411-protocols-impl-duplicate.sx:20:17
+  --> /Users/agra/projects/sx/examples/0411-protocols-impl-duplicate.sx:23:17
    |
-20 |   w : Wrap = xx 7;
+23 |   w : Wrap = xx 7;
    |                 ^

examples/expected/0763-modules-import-type-non-transitive.exit

@@ -0,0 +1 @@
+1

examples/expected/0763-modules-import-type-non-transitive.stderr

@@ -0,0 +1,5 @@
+error: type 'COnly' is not visible; #import the module that declares it
+  --> /Users/agra/projects/sx/examples/0763-modules-import-type-non-transitive.sx:17:9
+   |
+17 |     x : COnly = .{ v = 5 };
+   |         ^^^^^