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>
Co-Authored-By: Andrew Kesterson <andrew@aklabs.net>

include/akbasic/runtime.h

@@ -373,6 +373,37 @@ akerr_ErrorContext AKERR_NOIGNORE *akbasic_runtime_start(akbasic_Runtime *obj, i
  */
 akerr_ErrorContext AKERR_NOIGNORE *akbasic_runtime_load(akbasic_Runtime *obj, const char *source);
 
+/**
+ * @brief Find or create a variable in the script's outermost scope.
+ *
+ * **This is the entry point a host exchanging values with a script should use.**
+ * akbasic_environment_get() lands a new variable in whatever scope is active,
+ * and a script suspended part-way through a bounded akbasic_runtime_run() is
+ * usually inside a `FOR` or `GOSUB` body -- so the script reads the value
+ * correctly inside the loop and gets `0` immediately after it, with nothing
+ * raised anywhere. Reaching for the root by hand does not help either: that
+ * function only auto-creates in the *active* environment, so with a child active
+ * it returns NULL through `dest` without raising and an unchecked host
+ * dereferences it.
+ *
+ * Here the root is found by walking `obj->environment` to the environment with
+ * no parent, and the variable is created there unconditionally. That is the
+ * right answer inside a user function's scope too: a funcdef's environment is
+ * initialized with the caller's as its parent, so the walk terminates at the
+ * same root.
+ *
+ * Seeding before akbasic_runtime_start() and reading after the script stops both
+ * still work through this, so a host has no reason to use anything else.
+ *
+ * @param obj Object to initialize, inspect, or modify.
+ * @param name Variable name including its type suffix -- `A#`, `B%`, `C$`.
+ * @param dest Output destination populated by the function; never NULL on success.
+ * @return `NULL` on success, otherwise an error context owned by the caller.
+ * @throws AKERR_NULLPOINTER When any argument is NULL, or the runtime has no environment.
+ * @throws AKBASIC_ERR_BOUNDS When the name is too long, or no variable slot is free.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *akbasic_runtime_global(akbasic_Runtime *obj, const char *name, akbasic_Variable **dest);
+
 /* --- Internal API: exposed for the scanner, parser and verb handlers only. --- */
 
 /**