remove undefined global constraint

TODO.md

@@ -177,7 +177,7 @@
   - future direction: generalize toward Zig-style arbitrary pointer-result casts once casts have a broader result-type story
 
 12. `undefined` as inspired by zig (implemented):
-  - allow mutable local declarations with `undefined`
+  - allow mutable local and global declarations with `undefined`
     - undefined values are assigned a poison value (0xaa...)
     - allows for something like:
       ```
@@ -696,6 +696,7 @@
     and inferred array counts may resolve through the existing inference fixpoint, but
     the final type must be concrete runtime storage
   - emit source-defined mutable globals as writable globals, not constants
+  - allow `undefined` initializers for runtime storage initialized explicitly by a function
   - allow assignment, address-taking, field/index mutation, and pointer passing under
     the same mutability rules as other writable locations
   - keep mutable globals invalid in comptime evaluation; `$global_var` and writes from