bug: `using std` causes misleading error for undefined types like i512

[SchoolyB]

Summary

When any using directive is present, undefined types that look like sized integers (e.g., i512) produce a misleading "type mismatch" error instead of "undefined type".

Steps to Reproduce

Without using (correct behavior):

do main() {
    temp x i512 = 123
}

error[E3008]: undefined type 'i512'
  --> file.ez:2:10
   |
2  |     temp x i512 = 123
   |          ^ type is not defined

With any using directive (incorrect behavior):

import @std
using std

do main() {
    temp x i512 = 123
}

error[E3001]: type mismatch: cannot assign int to i512
  --> file.ez:5:10
   |
5  |     temp x i512 = 123
   |          ^ types do not match

Additional Testing

• Happens with using std, using crypto, using math - any using directive
• Does NOT happen with just import @module (no using)
• Other invalid types like foobar, i7, i100, i1024 correctly show "undefined type" even with using
• Issue appears specific to i512 pattern

Expected Behavior

Both cases should show error[E3008]: undefined type 'i512'.

Root Cause

The using directive appears to affect type resolution in a way that causes i512 to be treated as a valid type instead of undefined. This may be related to #909 (using directive scope issues).

Environment

• EZ version: dev (tested on main branch)