xppc: map inverter standby to OL

[user01010111]

Summary

Fixes #3512.

Line-interactive XPPC devices can report standBy(8) while utility
power continues to supply the load and only the inverter is idle. The
existing OFF mapping therefore makes upsmon treat a healthy online
UPS as not supplying power.

This changes the mapping to OL, clarifies the distinction between
inverter standby and UPS/output standby, bumps XPPC_MIB_VERSION to
0.42, and documents the correction in NEWS.adoc.

Issue #3512 reports that this mapping produces OL on utility power
and retains the expected OB/LB transitions on a Vertiv/Liebert PSI
PS1500RT3-230. This exact candidate has not been re-tested on that
hardware, so reporter confirmation remains the runtime acceptance
boundary.

Validation

• GCC 16.2.1 hard warnings with -Werror: complete snmp-ups and
  top-level builds passed.
• Clang 22.1.8 CI-style warnings with -Werror: complete snmp-ups
  build passed.
• The changed XPPC object passed Clang -Weverything -Werror.
• make stylecheck passed.
• The compiled lookup table reports value 8 as OL and XPPC MIB
  version 0.42.
• The configured test suite passed 9/9 with loopback sockets available.
• make -j8 SPELLCHECK_ERROR_FATAL=no check passed.
• make SPELLCHECK_ERROR_FATAL=no distcheck-light passed, including
  archive, build, check, install, uninstall and distclean.
• The candidate introduces no new spellcheck finding.
• Physical Liebert PSI confirmation remains pending.
• Upstream CI remains pending.

The full-driver Clang -Weverything -Werror probe encountered nine
existing float-conversion diagnostics in unchanged snmp-ups.c. The
changed XPPC object passed that stricter probe, and the project’s
CI-style Clang build passed.

General C checklist

• The change is one cohesive existing-subdriver correction.
• The XPPC MIB version was bumped.
• The mapping follows NUT’s established OL meaning.
• Coding style, ASCII portability and distribution checks passed.
• NEWS.adoc was updated.

AI assistance

OpenAI Codex gpt-5.6-sol was used for planning, repository analysis,
implementation, review, drafting and validation, including controller
and delegated work. The human contributor reviewed the change and
remains responsible for it.

[github-actions]

A ZIP file with standard source tarball and another tarball with pre-built docs for commit 93b1b2f is temporarily available: NUT-tarballs-PR-3600.zip.

[jimklimov]

Thanks!

[AppVeyorBot]

✅ Build nut 2.8.5.5165-master completed (commit 0b696b105e by @)

• artifacts
• A 7-zip archive with built NUT for Windows debug binaries is temporarily available: NUT-for-Windows-x86_64-SNAPSHOT-2.8.5.5165-master.7z