Fix premature submission of incomplete forms with unknown packages

When read-from-string hits a reader-error (e.g. unknown package symbol),
form-complete-p was returning T unconditionally, causing incomplete forms
to be submitted. Now checks paren balance on reader errors so unbalanced
forms correctly allow multiline continuation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>