refactor(ffi-linkage): Phase 9.3 — rename *-foreign* example files → extern/runtime names

git-mv the 10 foreign-named example families to extern/runtime-class names + update
every #import/#include/#source ref, stale comment ref, and the 1172 stderr snapshot
(path + 'extern symbol' message). Renames: 0729…-foreign→…-extern, 1172-diagnostics-
foreign-symbol-conflict→…-extern-symbol-conflict, 1205/1207 ffi-foreign-global→
ffi-extern-global, 1216/1217 ffi-…-foreign-(in-method|result-chain)→…-extern-…,
1219-ffi-foreign→1219-ffi-extern, 1306 objc-foreign-class-chained→objc-runtime-class-
chained, 1318 objc-property-foreign→objc-property-extern-class. DEDUP: deleted
1218-ffi-foreign-cvariadic (identical to 1229-ffi-extern-cvariadic; updated 1229's
twin ref) + the orphaned 1620 dir. Also purged editors/vscode tmLanguage (#foreign
dropped from the directive highlighter) + 1220.h/issues-0030.sx comment refs. Suite
green (644 corpus / 443 unit, 0 failed).

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

@@ -0,0 +1,5 @@
+// One of two flat authors of `absval`, a `extern` libc binding. A consumer
+// flat-importing BOTH must NOT see this as an ambiguous bare-call collision —
+// extern authors are never rerouted by the bare-call resolver, so the call
+// falls to the existing first-wins extern dispatch.
+absval :: (n: i32) -> i32 extern libc "abs";

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

@@ -0,0 +1,2 @@
+// The second flat author of `absval` — the identical `extern` libc binding.
+absval :: (n: i32) -> i32 extern libc "abs";