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/0076-stack-struct-addrof-passed-by-value.md

@@ -38,7 +38,7 @@
 > name span in the AST (`VarDecl.name_span`, `DestructureDecl.name_spans`,
 > `IfExpr`/`WhileExpr.binding_span`, `ForExpr.capture_span`/`index_span`,
 > `MatchArm.capture_span`, `CatchExpr`/`OnFailStmt.binding_span`,
-> `Protocol`/`ForeignMethodDecl.param_name_spans`), populated by the parser at
+> `Protocol`/`RuntimeMethodDecl.param_name_spans`), populated by the parser at
 > each binding site. `checkBindingNames` passes that span to the diagnostic, so
 > the caret underlines the offending identifier itself instead of the enclosing
 > statement / `if` / `match` / `protocol` / `#objc_class` block. No call site