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/1229-ffi-extern-cvariadic.sx

@@ -1,6 +1,5 @@
 // `extern` C-variadic tail: a trailing `..args: []T` on an `extern` fn
-// maps to the C calling convention's `...`, exactly like its `extern`
-// twin (example 1218). Extras at the call site pass through the variadic
+// maps to the C calling convention's `...`. Extras at the call site pass through the variadic
 // slot with standard default argument promotion (i8/i16/bool → i32,
 // f32 → f64), NOT packed into an sx slice.
 //