fix(lua-5.4): 1-arg os.difftime in 6 imported scripts

[Aybook]

Sweep of an audit miss. The initial pattern check across PRs #1, #3, and #5 missed the nested-paren form `os.difftime( os.time() - X )` because the regex didn't see through the inner parentheses. The issue surfaced on the announcements batch (#5) and got fixed there for three scripts. This PR closes the rest of the gap on master.

The bug

Pattern: `os_difftime( os_time() - SOMETIME ) >= delay` inside onTimer handlers.

Lua 5.4 strict-checks `os.difftime` arity (must be 2 args). The upstream wrote one expression-arg pattern that reduced to a single number and 5.4 errors at runtime: `bad argument #2 to 'difftime' (number expected, got no value)`.

The smoke harness has been silently passing because its ~5-10s window doesn't always trigger an onTimer firing while the listener is bad-shaped. On a real deploy it would error on the first tick.

Fix

Replaced `os_difftime( os_time() - X )` with the equivalent direct subtraction `(os_time() - X)`. Same pattern that landed in the announcements PR (#5).

Affected files

File	Line	Source PR
`scripts/cmd_pm2offliners/cmd_pm2offliners.lua`	278	#3
`scripts/etc_banner_mod/etc_banner_mod.lua`	143	#5
`scripts/etc_EventAnnouncer/etc_EventAnnouncer.lua`	160, 180	#5
`scripts/etc_mainecho/etc_mainecho.lua`	121	#1
`scripts/etc_messenger/etc_messenger.lua`	133	#1
`scripts/etc_wunschbrett/etc_wunschbrett.lua`	134	#1

7 occurrences across 6 files. +7 / -7 lines.

Test plan

• Smoke harness against `luadch-ng/luadch` v3.1.1 with all merged batches plus this fix dropped into the install: 10 / 10 PASS.
• Multiline-aware audit on all imported scripts confirms no remaining 1-arg `os.difftime` patterns. The only remaining `os.difftime` calls are legitimate 2-arg ones (`ptx_rotatingtopic.lua`, three sites in `ptx_tophubbers.lua`).

Future-proofing

For upcoming import batches I'm using a multiline-aware audit (awk that skips block-comment regions) that walks every `os.difftime` call site and inspects the full arglist between the matched parens, instead of a flat regex.