refactor(ffi-linkage): Phase 9.3-src — purge 'foreign' from src/ comments + a user-facing diagnostic

Reword every 'foreign' comment to the extern/runtime-class vocabulary matching the
renamed identifiers (foreign call→extern call, foreign class→runtime class, foreign
path→runtime path, the #foreign-literal comment mentions → extern, etc.). Also fixes
two USER-FACING issues: the 'expected … #foreign … after type annotation' parse error
no longer advertises the removed keyword, and the Android 'no #jni_main' help
diagnostic now shows '#jni_class(…) extern' instead of the rejected '#foreign
#jni_class'. Removed the now-dead prefix-#foreign-vs-postfix conflict branch in
parseRuntimeClassDecl (the caller rejects #foreign before it runs).

src/ now contains 'foreign' ONLY in the hash_foreign token machinery + its 4
rejection messages — the deprecation mechanism (kept per the 9.0 recommendation; the
message MUST name #foreign to guide migration). Snapshot-neutral; suite green
(646 corpus / 444 unit, 0 failed).

src/ir/host_ffi.zig

@@ -77,7 +77,7 @@ pub fn callVoidRet(symbol: *anyopaque, args: []const usize) !void {
 }
 
 // ── Variadic cdecl dispatch ─────────────────────────────────────────
-// For foreign functions declared with `args: ..T` (C-variadic, e.g.
+// For extern functions declared with `args: ..T` (C-variadic, e.g.
 // libc `open(path, flags, ...)`). The trailing args must be passed
 // through the C-variadic ABI — arm64 places them on the stack rather
 // than in argument registers. Calling a variadic function as if it