Add SX test harness: mock IO platform for testing components

spec/harness.sx — spec-level test harness with:
- Mock platform (30+ default IO mocks: fetch, query, DOM, storage, etc.)
- Session management (make-harness, harness-reset!, harness-set!/get)
- IO interception (make-interceptor, install-interceptors)
- IO log queries (io-calls, io-call-count, io-call-nth, io-call-args)
- IO assertions (assert-io-called, assert-no-io, assert-io-count, etc.)

15 harness tests passing on both OCaml (1116/1116) and JS (15/15).
Loaded automatically by both test runners.

MCP tool: sx_harness_eval — evaluate SX with mock IO, returns result + IO trace.

The harness is extensible: new platforms just add entries to the platform dict.
Components can ship with deftest forms that verify IO behavior against mocks.
Tests are independent objects that can be published separately (by CID).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/bin/run_tests.ml

@@ -10,12 +10,7 @@
       dune exec bin/run_tests.exe -- test-primitives  # specific test
       dune exec bin/run_tests.exe -- --foundation     # foundation only *)
 
-module Sx_types = Sx.Sx_types
-module Sx_parser = Sx.Sx_parser
-module Sx_primitives = Sx.Sx_primitives
-module Sx_runtime = Sx.Sx_runtime
-module Sx_ref = Sx.Sx_ref
-module Sx_render = Sx.Sx_render
+(* Modules accessed directly — library is unwrapped *)
 
 open Sx_types
 open Sx_parser
@@ -273,7 +268,7 @@ let make_test_env () =
     | _ -> raise (Eval_error "append!: expected list and value"));
 
   (* --- HTML Renderer (from sx_render.ml library module) --- *)
-  Sx.Sx_render.setup_render_env env;
+  Sx_render.setup_render_env env;
 
   (* Stubs needed by adapter-html.sx when loaded at test time *)
   bind "set-render-active!" (fun _args -> Nil);
@@ -734,6 +729,14 @@ let run_spec_tests env test_files =
   Printf.printf "\nLoading test framework...\n%!";
   load_and_eval framework_path;
 
+  (* Load test harness (mock IO platform) *)
+  let harness_path = Filename.concat (Filename.concat project_dir "spec") "harness.sx" in
+  if Sys.file_exists harness_path then begin
+    Printf.printf "Loading test harness...\n%!";
+    (try load_and_eval harness_path
+     with e -> Printf.eprintf "Warning: harness.sx: %s\n%!" (Printexc.to_string e))
+  end;
+
   (* Load modules needed by tests *)
   let spec_dir = Filename.concat project_dir "spec" in
   let lib_dir = Filename.concat project_dir "lib" in