test: pin C function pointers into sx (PLAN-HTTPZ C2 — pre-existing)

Both halves of the C2 contract already work in JIT and AOT; these
examples pin them. 1635: libc qsort drives an sx callconv(.c)
comparator passed by name as a typed fn-pointer param. 1636: a real
pthread enters sx through a callconv(.c) entry, fabricates its own
Context (push Context with a local GPA), and runs default-conv sx code
that allocates through it — the re-entry contract std.thread (S6)
stands on. Also unblocks the sqlite callback APIs (hooks/UDFs) left
unbound by design in P5.1.

examples/expected/1635-cfnptr-qsort.stdout

@@ -0,0 +1 @@
+qsort via sx comparator ok