Add akbasic_runtime_global: a host variable lands in the script's root scope

A host creating a variable while a script was suspended got it in whatever
scope was active -- usually a FOR or GOSUB body -- and it died when the body
popped, silently. Reaching for the root by hand returned NULL without raising,
because environment_get only auto-creates in the active environment.

Both are still true of environment_get, which is correct for what the
interpreter uses it for. The README and the example now point somewhere else.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -238,6 +238,7 @@ set(AKBASIC_TESTS
   error_codes
   grammar_leaves
   graphics_verbs
+  hostvars
   input_verbs
   numeric_contract
   parser_commands