Save dropped items

[Makkkkus]

Dropped items are not saved by the server when closing. All dropped items should be saved.

[beleata]

There is a fix for this in #5, along with an honest note on what it does not cover.

The reason dropped items were not saved is that such an item belongs to no vessel
and to nobody's inventory — it rides on its own Pivot, and nothing reachable from
Persistence ever sees it. It now records which vessel the item was released in,
and puts it back on a pivot there when the world loads. Verified across restarts:
written to the save, restored, saved again.

What the PR does not fix: a joining client is told about these items but does
not draw them. The server has the item about 0.2 m from the station's centre while
the client draws it 25 m away, where it was dropped — the two measure in different
frames and nothing converts between them. That looks like #1's territory rather
than something to patch around here, so we left it, and wrote down the six
approaches we tried and why each failed.

Worth knowing before anyone tests this: world saving itself has been commented out
since c89a61f (2023-07-23), so the entire load path had never run. Several crashes
were waiting behind it, including one on every player death. Those are in the same
PR.