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/0121-pack-fn-alias-unresolved.md

@@ -108,5 +108,5 @@ Context: BLOCKS the std.sx-as-pure-re-exports restructure — `print` /
 unblocked by 0120; this is the remaining known gap. Still unprobed
 for the restructure (next session, after this fix): protocol aliases
 (`Allocator`, parameterized `Into`), `#builtin` decl aliases
-(`size_of`, `out`, `string :: []u8`), `#foreign` decl aliases
+(`size_of`, `out`, `string :: []u8`), `extern` decl aliases
 (`memcpy`).