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/target.zig

@@ -535,7 +535,7 @@ pub fn link(allocator: std.mem.Allocator, io: std.Io, output_obj: []const u8, ex
             try argv.append(allocator, try std.fmt.allocPrint(allocator, "-L{s}", .{lp}));
         }
 
-        // Auto-detect host OS library paths when linking foreign libraries
+        // Auto-detect host OS library paths when linking external libraries
         if (libraries.len > 0 and target_config.triple == null) {
             for (host_lib_paths) |path| {
                 try argv.append(allocator, try std.fmt.allocPrint(allocator, "-L{s}", .{path}));