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

DOCFIX-173: full-project sweep -- script logic bugs + security hardening ...

Four confirmed logic bugs (unguarded pipelines/function calls under
set -e+pipefail), all independently reproduced in isolation before being
trusted -- including catching that a first proposed fix for one of them
was itself wrong: `|| true` after a failing pipeline under pipefail
silently discards PIPESTATUS, which would have made phase-06-capi-stack's
own gate check never fire again. The correct `if pipeline; then :; fi`
idiom was verified via reproduction before committing.

- carve-host-interfaces.sh: emit()'s unguarded `return 1` killed apply-mode
  runs on the first MAAS error, before the FATAL-accumulator summary
  could print.
- phase-06-capi-stack.sh: run_step's PIPESTATUS read was dead code.
- juju-spaces-check.sh, osd-blank-check.sh: one unguarded display-only
  pipe each, safely `|| true`-able.
- ledger-scan.sh: DOCFIX/BUNDLEFIX next-free regex used a fixed {3} not
  {3,} -- the same bug class already fixed once for the D-series, never
  mirrored here. Dormant, not yet reachable, but a real landmine.
- dc-dc-rbd-mirror.sh: hardened the bootstrap token's default path
  (random suffix + immediate chmod 600, was a predictable /tmp filename
  with no permission hardening).

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