failsafe boot mode

internal/engine.cm

@@ -68,6 +68,8 @@ function use_core(path) {
   var result = null
   var script = null
   var ast = null
+  var mcode_path = null
+  var mcode_blob = null
 
   // Build env: merge core_extras
   env = {use: use_core}
@@ -78,6 +80,26 @@ function use_core(path) {
   var mach_blob = null
   var source_blob = null
 
+  // Check for pre-compiled .cm.mcode JSON IR (generated by regen)
+  mcode_path = core_path + '/boot/' + replace(path, '/', '_') + '.cm.mcode'
+  if (fd.is_file(mcode_path)) {
+    mcode_blob = fd.slurp(mcode_path)
+    hash = content_hash(mcode_blob)
+    cached_path = cache_path(hash)
+    if (cached_path && fd.is_file(cached_path)) {
+      result = mach_load(fd.slurp(cached_path), env)
+    } else {
+      mach_blob = mach_compile_mcode_bin('core:' + path, text(mcode_blob))
+      if (cached_path) {
+        ensure_build_dir()
+        fd.slurpwrite(cached_path, mach_blob)
+      }
+      result = mach_load(mach_blob, env)
+    }
+    use_cache[cache_key] = result
+    return result
+  }
+
   // Compile from source .cm file
   var file_path = core_path + '/' + path + MOD_EXT
   if (fd.is_file(file_path)) {