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/inst.zig

@@ -498,7 +498,7 @@ pub const Function = struct {
     linkage: Linkage = .internal,
     call_conv: CallingConvention = .default,
     source_file: ?[]const u8 = null,
-    /// Variadic tail at the IR signature level. Only `#foreign` decls reach
+    /// Variadic tail at the IR signature level. Only `extern` decls reach
     /// IR with this set — sx-side `..T` params are slice-packed before
     /// lowering, so anything that survives is the C calling convention's
     /// `...`. emit_llvm passes `is_var_arg=1` to `LLVMFunctionType`; call