An empty archived section still draws the sidebar's section border, and re-fetches the roster every 10 s on top of the structure refresh #818

What is wrong

Two costs the archived section still has when it has nothing to show (plugins/tree/client.js, added by #34 / f20cfa37).

1. An empty section still draws the sidebar's separator. The slot host wraps every flybot.sidebar.section entry in div.bt-flybot-section (core/src/client/flybot-slots.js:113-116), and that class is styled unconditionally (:292): margin-top: 8px; border-top: 1px solid …; padding-top: 6px. ArchivedView returns null at client.js:462 when bots.length === 0, which removes the header and the rows but not the wrapper. The reorganisation notice (:516) also renders nothing when it has none, so a fresh install draws two 1 px lines 8 px apart with nothing between them. Tree: a reachable Archived group in the sidebar, and one group for a concurrent burst #810's acceptance says "no header, no border, no empty group"; the third part does not hold, and the test renders the component alone — it has never rendered the slot wrapper.
2. The section re-fetches the roster every 10 s on top of the refresh that already runs. makeArchivedSection (:485) installs its own setInterval(refresh, REFRESH_MS) with REFRESH_MS = 10000 (:277), while registerStructure (:352) already calls flybot.bots() on the same interval. With both registered the roster is fetched twice per 10 s where once is enough.

Expected behaviour

An archived section with no archived Bots leaves no trace in the sidebar — no border, no margin, no padding — and, with both entries registered, the roster is fetched once per refresh tick. With archived Bots present the section looks and behaves exactly as it does today.

Acceptance

1. A test renders the slot host (FlyBotSections / the sidebar path), not the component on its own, with an entry whose component returns null, and asserts no visible separator: either no wrapper element is produced, or the wrapper matches :empty and its computed style is border-top: 0, margin-top: 0, padding-top: 0. It fails on today's code and passes after the fix.
2. The same test or a sibling renders the archived entry with N > 0 and asserts the header Archived (N), the rows, and one separator are present (the fix must not hide a section that has content).
3. Counting flybot.bots() across one 10 s tick with both the structure entry and flybot-tree-archived registered: exactly one call (today two). Rows still update after an archived field change and after bot.removed, and an expanded section stays expanded across a tick.
4. With N = 0 nothing is drawn at all; with N = 1 the header reads Archived (1).
5. npm test in core and plugins/tree green; plugin-imports and the arch-docs check green.

Kill conditions

• Hiding the wrapper with display: none whenever it is empty while also hiding a wrapper that has rows — the rule must key on emptiness, not on the section id.
• Removing the wrapper for every section, which breaks the spacing of sections that do have content.
• Fixing the border in the tree's own CSS while the host rule stays unconditional: the next null-rendering section draws the line again.
• "Fixing" the duplicate fetch by lengthening the interval or by dropping the field/removal updates.
• Touching Tree reorg must MOVE/reparent bots; never rename to 归档- and recreate #452's include() or moving archived Bots back into the Bot list.

Out of scope

• The reorganisation notice's content, Undo and Dismiss.
• The optional flybot-tree section at :126 (sidebarSection: true only).
• The reorg notice's own null state: same wrapper problem, fixed by the same host rule; no separate change to that component.