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).
A trailing `..args: []T` on an `extern` fn must map to the C `...` tail
like its `#foreign` twin (example 1218). Today the variadic handling in
both declareFunction (is_variadic drop) and packVariadicCallArgs
(call-site early-out) is gated on `#foreign` only, so a variadic
`extern` keeps the trailing slice param and slice-packs the extras —
garbage at the C ABI (probe: sum_ints(3,10,20,30) → 53316585, not 60).
Example 1229 pins the DESIRED correct output; the next commit extends
both gates to cover extern and greens it. Prerequisite for migrating the
fn-decl `#foreign` path onto `extern`.
645 corpus (1229 xfail), 444 unit.