fix(internaldata): stamp ended_at when the territory-drift check abandons an encounter

[nxships]

UpsertSightingAsync's territory-drift branch nulled mCurrentEncounterId
without stamping ended_at, unlike OnTerritoryChanged and the world-drift
branch immediately below it. A missed or late TerritoryChanged therefore
left the row dangling at ended_at IS NULL until the next session's orphan
sweep - and when that reload happened to land in the same territory, pass
1's unbounded crash-recovery branch resumed it and welded two unrelated
visits into one encounter with a bogus duration.

Both drift branches now feed a shared driftStampId slot. They are mutually
exclusive by construction: the territory branch nulls mCurrentEncounterId,
so the world branch's mCurrentEncounterId is { } drifted pattern can no
longer match afterwards.

Also add a one-shot warning to the watcher's ProcessAsync IsStopping bail.
That path returning silently is why the lifetime-token regression fixed in
NexusKit (Dalamud's load-timeout token being treated as the plugin
lifetime) went unnoticed for weeks: observation persistence died 60s into
every session, Recent and ObservationProcessed froze, and the nearby-player
list silently stopped growing with nothing in the log. On a legitimate
unload the warning fires at most once during teardown.

Docs: correct the "plugin unload leaves the encounter open" claim - the
Stopping transition now closes it cleanly, so only a hard crash leaves
ended_at NULL - and document both drift checks plus the
COUNT(*) WHERE ended_at IS NULL invariant.