Fix st_cast empty geometry split ordering

[kendonB]

Summary

• preserve empty geometries when st_cast splits multiparts into multiple rows
• rebuild output in original order and keep ids so attributes repeat correctly

Testing

• devtools::load_all('.')
• ran user repro (MULTIPOLYGON with empty) -> returns 5 rows with correct names/bboxes

[kendonB]

FYI this was all gpt-5.1-codex-max-xhigh (including the PR)

[edzer]

That's a lot of code. Could you provide a very small example of what this code tries to solve?

[rsbivand]

Testing should also include full reverse dependency checks of all reverse depencencies, with comparative timings and memory profiling.

[kendonB]

Sorry I missed the reference to the issue which contains the small example of what this code tries to solve #2555

[kendonB]

Testing should also include full reverse dependency checks of all reverse depencencies, with comparative timings and memory profiling.

Running this now with r-lib/revdepcheck