Add py_Callbacks.open_file hook for embedder file-access policy

Adds an optional open_file callback to py_Callbacks, consulted before
script-reachable file operations:

  - io.FileIO(path, mode): called with the fopen mode string before the open
  - os.remove(path): called with the literal "delete"

Returning false rejects the operation and raises OSError; returning true
(or leaving the callback NULL, which is the default) allows it. Existing
embedders are unaffected -- the default is NULL and behavior is unchanged.

This lets an embedder enforce its own path policy using its own
canonicalization instead of reimplementing path checks inside the VM.

include/pocketpy/pocketpy.h

@@ -78,6 +78,13 @@ typedef struct py_Callbacks {
     PY_MAYBENULL void (*gc_mark)(void (*f)(py_Ref val, void* ctx), void* ctx);
     /// Used by `PRINT_EXPR` bytecode.
     PY_MAYBENULL bool (*displayhook)(py_Ref val) PY_RAISE;
+    // open_file hook contributed by fdtd.io (Hector), 2026.
+    /// Consulted before a script-reachable file operation. `path` is the target path;
+    /// `mode` is the fopen mode string for `io.FileIO`, or the literal "delete" for
+    /// `os.remove`. Return true to allow the operation, false to reject it (the binding
+    /// then raises OSError). NULL (the default) allows everything, so existing embedders
+    /// are unaffected. Lets an embedder enforce its own path policy.
+    PY_MAYBENULL bool (*open_file)(const char* path, const char* mode);
 } py_Callbacks;
 
 /// A struct contains the application-level callbacks.

src/interpreter/vm.c

@@ -92,6 +92,7 @@ void VM__ctor(VM* self) {
     self->callbacks.print = pk_default_print;
     self->callbacks.flush = pk_default_flush;
     self->callbacks.getchr = pk_default_getchr;
+    self->callbacks.open_file = NULL;
 
     self->last_retval = *py_NIL();
     self->unhandled_exc = *py_NIL();

src/modules/os.c

@@ -71,6 +71,10 @@ static bool os_remove(int argc, py_Ref argv) {
     PY_CHECK_ARGC(1);
     PY_CHECK_ARG_TYPE(0, tp_str);
     const char* path = py_tostr(py_arg(0));
+    // open_file policy hook: "delete" pseudo-mode for os.remove.
+    if(pk_current_vm->callbacks.open_file && !pk_current_vm->callbacks.open_file(path, "delete")) {
+        return OSError("os.remove not permitted: '%s'", path);
+    }
     int code = remove(path);
     if(code != 0) {
         const char* msg = strerror(errno);
@@ -111,6 +115,11 @@ static bool io_FileIO__new__(int argc, py_Ref argv) {
     io_FileIO* ud = py_newobject(py_retval(), cls, 0, sizeof(io_FileIO));
     ud->path = py_tostr(py_arg(1));
     ud->mode = py_tostr(py_arg(2));
+    // open_file policy hook: consulted with the fopen mode string before the open.
+    if(pk_current_vm->callbacks.open_file &&
+       !pk_current_vm->callbacks.open_file(ud->path, ud->mode)) {
+        return OSError("file open not permitted: '%s' (mode '%s')", ud->path, ud->mode);
+    }
     ud->file = fopen(ud->path, ud->mode);
     if(ud->file == NULL) {
         const char* msg = strerror(errno);