appIndicator: Guarantee the last icon update paints

[everyx]

Pixmap-based StatusNotifierItems that update in bursts, or that re-register (sleep/resume, app or shell restarts), can leave the tray with a blank slot that never heals and logs nothing: if the settled fetch is undisplayable -- an empty pixmap because the signal arrived before the data, or a failed Get during re-registration flicker -- no later signal ever repaints it. Fresh registrations hit the same hole at first paint, when the name and object exist before the app populates its pixels.

Observed instance: WeChat, China's national-level Qt chat app, is pixmap-only (empty IconName). It fires NewIcon bursts while its IconPixmap flaps between the real icon and fully transparent 16x16 frames; after sleep/resume it re-registers under a new bus name and first-paints before its pixels land.

Fix with delivery guarantee, under these principles:

• Faithful renderer: any pixels the app exports are painted as-is, however suspicious (even fully transparent). Guessing intent at this layer breaks real apps (cf. fcitx5's deliberate transparent overlay, upstream Pixmap icon is not reset when icon transition from iconpixmap to icon #468). Transparent overlays are never touched.
• Exception fallback only: keep the old icon solely when there is literally nothing to display (empty/failed/malformed fetch).
• Last-wins delivery: after a refresh leaves a base icon family (Icon/AttentionIcon) undisplayable, re-fetch with bounded backoff once the burst settles (350ms, 2s, 8s; max 3 attempts). Also covered is the first paint of fresh registrations, via the ready-transition hook. Generation-guarded and bounded, so a stale retry can never overwrite newer state (the Possible stale icon race: cached icon update does not cancel an earlier asynchronous load #663/appIndicator: Cancel pending icon loads on cache hit #664 race class). AttentionIcon participates only while NeedsAttention; an empty attention icon is otherwise healthy.

Refs #468 (pixmap/content duality), #663/#664 (async icon loading races).