refactor(ffi-linkage): Phase 9.3/9.4 — purge 'foreign' from issues/*.md; GATE PASS

Rewrote 20 issue writeups to the extern/runtime-class vocabulary (#foreign→extern,
foreign_class_map→runtime_class_map, parseForeignClassDecl→parseRuntimeClassDecl,
findForeignMethodInChain→findRuntimeMethodInChain, dedupeForeignSymbol→
dedupeExternSymbol, is_foreign_c_api→is_extern_c_api, stale filename refs to the
renamed examples, foreign-class→runtime-class, bare foreign→extern). Renamed
issues/0043-…-foreign-class-…→…-runtime-class-….

PHASE 9 COMPLETE — 9.4 GATE PASSES: zero 'foreign' across src/library/examples/
issues/docs/editors/specs/readme/CLAUDE, excluding only the SQLite API constant
SQLITE_CONSTRAINT_FOREIGNKEY + vendored sqlite3.c/.h (upstream third-party).
Suite green (644 corpus / 443 unit, 0 failed).

issues/0030-extern-global-declarations.md

@@ -9,7 +9,7 @@
 
 Support an `extern G : T;` top-level form so a global **defined** in one sx
 source file can be **referenced** from another without threading it through
-parameters — mirroring how `#foreign` function declarations work (declared in one
+parameters — mirroring how `extern` function declarations work (declared in one
 place, defined elsewhere, resolved at link time).
 
 ```sx
@@ -28,8 +28,8 @@ load :: (self: *ChessPieces, path: [:0]u8) {
 
 Today `pieces.load` takes `has_gpu: bool, gpu: GPU` params and `main.sx` threads
 them through; cross-file `extern` globals would drop that ceremony. Distinct from
-the existing `name : T #foreign;` form (an *external C* data symbol from
-libsystem etc. — see `examples/1205-ffi-foreign-global.sx`); this request is for
+the existing `name : T extern;` form (an *external C* data symbol from
+libsystem etc. — see `examples/1205-ffi-extern-global.sx`); this request is for
 sx-defined globals shared across sx modules.
 
 ## Reproduction