Fix WASM browser click handlers: 8 bugs, 50 new VM tests

The sx-get links were doing full page refreshes because click handlers
never attached. Root causes: VM frame management bug, missing primitives,
CEK/VM type dispatch mismatch, and silent error swallowing.

Fixes:
- VM frame exhaustion: frames <- [] now properly pops to rest_frames
- length primitive: add alias for len in OCaml primitives
- call_sx_fn: use sx_call directly instead of eval_expr (CEK checks
  for type "lambda" but VmClosure reports "function")
- Boot error surfacing: Sx.init() now has try/catch + failure summary
- Callback error surfacing: catch-all handler for non-Eval_error exceptions
- Silent JIT failures: log before CEK fallback instead of swallowing
- vm→env sync: loadModule now calls sync_vm_to_env()
- sx_build_bytecode MCP tool added for bytecode compilation

Tests: 50 new tests across test-vm.sx and test-vm-primitives.sx covering
nested VM calls, frame integrity, CEK bridge, primitive availability,
cross-module symbol resolution, and callback dispatch.

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

hosts/ocaml/lib/sx_primitives.ml

@@ -362,6 +362,7 @@ let () =
     | [Macro _] | [Thunk _] | [Keyword _] | [Symbol _] -> Number 0.0
     | _ -> raise (Eval_error (Printf.sprintf "len: %d args"
         (List.length args))));
+  register "length" (Hashtbl.find primitives "len");
   register "first" (fun args ->
     match args with
     | [List (x :: _)] | [ListRef { contents = x :: _ }] -> x