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/0102-flat-import-same-signature-collision.md

@@ -43,7 +43,7 @@ collision:
   `resolveBareCallee(name, caller_file) -> .func(ResolvedAuthor) | .ambiguous |
   .none` (`src/ir/lower.zig`). It returns `.none` whenever the outcome would
   equal first-wins (single author, or own-author == winner), so every
-  single-author / local / parameter / std / qualified / foreign / generic /
+  single-author / local / parameter / std / qualified / extern / generic /
   builtin name resolves byte-for-byte as before. Only a genuine flat collision
   reroutes: own-author wins; else the caller's flat-reachable authors — `≥2`
   distinct → `.ambiguous` (loud "qualify the call" diagnostic), exactly one
@@ -85,7 +85,7 @@ fails on pre-fix code and passes after):
 - `0726-modules-flat-same-name-variadic` — per-source variadic packing.
 - `0728-modules-flat-same-name-paramtype` — per-source parameter target typing
   (value vs pointer param).
-- `0729-modules-flat-same-name-foreign` — same-name `#foreign` authors are NOT
+- `0729-modules-flat-same-name-extern` — same-name `extern` authors are NOT
   rerouted (non-plain authors keep first-wins).
 - `0730-modules-flat-same-name-default-arg` — per-source default-arg expansion.
 - `0731-modules-flat-same-name-closure` — per-source `closure(fn)` + bare