DOCFIX-174: fix self-inflicted false positive in ledger-scan.sh next-free scan ...

DOCFIX-173's own changelog narrated its {3}->{3,} regex fix using a literal
illustrative example ("DOCFIX-1004"), which the widened regex then matched
as a real assignment -- inflating DOCFIX next-free from 174 to 1005.
Reworded the illustrative examples in prose, added a reproduc-line
exclusion to nextfree_mentions() as defense-in-depth, and added a
regression fixture to the test harness (37/37 PASS).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QrcJx8TUar7pYAvpGJw57A