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/0100-cross-module-same-name-fn-lowering-collision.md

@@ -21,7 +21,7 @@ the same colliding entry.
    `ns.fn` — they just had nothing to find. `NamespaceDecl` carries the
    module's `own_decls` (populated in `imports.addNamespace`) so the
    registration covers authored decls, not transitive flat imports. Generic
-   / comptime / pack / foreign functions are excluded — they dispatch by
+   / comptime / pack / extern functions are excluded — they dispatch by
    monomorphization off the bare template name, not the plain
    `resolveFuncByName` path, so a qualified alias would strand their
    per-call type bindings. The qualified function is declared + lowered on
@@ -106,7 +106,7 @@ class can't diverge again. The fields the defer now restores on all paths:
 - `builder.current_block`
 - `builder.inst_counter`
 
-(The `current_foreign_class`, `jni_env_stack_base`, and pack-mono /
+(The `current_runtime_class`, `jni_env_stack_base`, and pack-mono /
 `inline_return_target` fields already had their own `defer`s and apply on all
 paths; they are unchanged.)