tree load: end a turn by its turnId, not by whichever start is oldest #696

Summary

The tree's load tracker cannot tell two open turns apart, so turn.end ends the oldest start. That is the one case where the number is supposed to help: a Bot with two turns open, the newer one finishing first, now reports the newer turn's age as "how long this Bot has been open" — the stall the tracker exists to surface (#624) reads as recent.

Where

plugins/tree/index.js:36-37 passes only the Bot id:

fb.onFeed('turn.start', e => { tree.noteTurnStart(e?.botId); refreshParentPrompt(e?.botId) }),
fb.onFeed('turn.end',   e => { tree.noteTurnEnd(e?.botId);   refreshParentPrompt(e?.botId) }),

The feed event itself carries turnId (core/src/client/flybot-feed.js, the turn.start / turn.end payloads), and plugins/tree/src/load.js keeps one start time per open turn with end(id) dropping the oldest — the shift is deliberate (a turn id is not passed in), but it means an out-of-order finish resets openFor() to the wrong start.

Change

• load.js: start(id, turnId) / end(id, turnId) store the id with the start time; end removes the matching turn when the id is known, and keeps the current shift-oldest behaviour when it is missing or does not match (unmatched fallback — old clients, a dropped turn.start).
• plugins/tree/index.js: pass e?.turnId on both feeds. openFor(id, now) and everything list_children returns keep their current shape.

Acceptance

• Two turns open (t1 at 00:00, t2 at 00:30), end(id, 't2') → openFor still reports the age of t1 (the older start), and of(id) is 1.
• end(id, 'nope') (unknown id) and end(id) (no id) each remove exactly one turn and leave the older start standing.
• Revert the id pairing (keep only the shift) → the first case goes red; node --test plugins/tree/test/ green at the tip.
• The openForMs / stalled wording from A Bot session stops reading its queue while still reporting load: 1 (Dev: 11 unread, zero progress) #624 is unchanged, including its documented "time-based hint" limit.

Tier: plugins/tree/** → T2 (PASS at the tip, then the queue).