Replace (?<!\d)/(?!\d) lookarounds on the digit block in _INN_RE with
(?<!\w)/(?!\w). The old (?<!\d) did not block alpha-prefixed tokens
(e.g. «ИНН ref7707083893»), and (?!\d) did not block alpha-suffixed
tokens (e.g. «ИНН 7707083893more»). \b is unsuitable here because
Python \w covers both letters and digits, so there is no \b boundary
between an alpha char and a digit char. The (?<!\w)/(?!\w) pair
correctly anchors the 10/12-digit INN block to non-word boundaries on
both sides. Context anchor and checksum gate from #1640 are unchanged.
Adds 6 regression tests covering: alpha-prefix, alpha-suffix,
embedded mid-token, 12-digit alpha-prefix, punctuation separator
(should match), and end-of-string (should match).
