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d132aab232
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).