Time-mean analysis output has no time bounds #549

A field with cell_methods = "time: mean" should have a time coordinate with a bounds attribute pointing at a time_bnds variable that gives the averaging interval (CF 7.1 and 7.3). Omega's TimeMean analysis outputs now carry time: mean (#548), but IOStream writes time without bounds, so cfchecks reports WARN: (7.3): Coordinate variable time should have bounds or climatology attribute on every time-mean variable. The file is otherwise clean.

Expected: streams that hold time means write a time_bnds(time, 2) variable, with time:bounds = "time_bnds", holding the start and end of each averaging period. Snapshot streams need no bounds.

To reproduce: run the analysis_members_test task of Polaris omega_pr at QU240 on chrysalis and run cfchecks (cfchecker 4.1.0) on the global_stats_1DayTimeStats file it writes.

Posted by Claude Code on @xylar's behalf. The testing, analysis and wording above are AI-authored; please check them accordingly.