refactor(ffi-linkage): Phase 8 pre-cutover — migrate multi-file example companions

The Phase 7 batches globbed top-level examples/*.sx and missed #foreign decls in
SUBDIRECTORY companion files. Migrate the 4 incidental fn decls (behavior-preserving):
0729/a.sx + b.sx (same-name 'absval' libc authors), 1617/c.sx (pcap_lib_version),
1623/mod.sx (unit_in_mod_answer) — all '#foreign LIB "csym";' → 'extern LIB
"csym";'. Parents all marker'd → corpus-validated; empty snapshot diff. Comment
lines left for Phase 9.3. Suite green (647/444).

examples/0729-modules-flat-same-name-foreign/a.sx

@@ -2,4 +2,4 @@
 // flat-importing BOTH must NOT see this as an ambiguous bare-call collision —
 // foreign authors are never rerouted by the bare-call resolver, so the call
 // falls to the existing first-wins foreign dispatch.
-absval :: (n: i32) -> i32 #foreign libc "abs";
+absval :: (n: i32) -> i32 extern libc "abs";

examples/0729-modules-flat-same-name-foreign/b.sx

@@ -1,2 +1,2 @@
 // The second flat author of `absval` — the identical `#foreign` libc binding.
-absval :: (n: i32) -> i32 #foreign libc "abs";
+absval :: (n: i32) -> i32 extern libc "abs";

examples/1617-modules-library-nested-namespace/c.sx

@@ -2,7 +2,7 @@
 #import "modules/std.sx";
 
 pcaplib :: #library "pcap";
-pcap_lib_version :: () -> ?cstring #foreign pcaplib "pcap_lib_version";
+pcap_lib_version :: () -> ?cstring extern pcaplib "pcap_lib_version";
 
 pcap_version :: () -> string {
     p := pcap_lib_version();

examples/1623-cimport-unit-in-aliased-module/mod.sx

@@ -5,6 +5,6 @@ cunit :: #import c {
     #source "inmod.c";
 };
 
-unit_in_mod_answer :: () -> i32 #foreign cunit "unit_in_mod_answer";
+unit_in_mod_answer :: () -> i32 extern cunit "unit_in_mod_answer";
 
 answer_via_mod :: () -> i32 { return unit_in_mod_answer(); }