penguinolog / urwid / 37029119262

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Stop CanvasCache._deps keeping dead widgets alive

store() added each dependant to _deps[child] and nothing removed it until
the child itself was invalidated, so wrappers rebuilt around a cached child
(and GridFlow display widgets rebuilt on every focus change) were never freed.

- Add _children, the reverse of _deps, and drop a widget's links when its
  last cached canvas is collected or it is invalidated, and remove a child
  from its dependants' links when the child's own last canvas is collected.
- store() adds links only after the canvas is cached, so a cleanup callback
  running mid-store cannot strip them or leave them for an uncached canvas.
- cleanup() of a replaced canvas no longer removes the live entry stored
  under the same key.
- Raise a new CanvasError on each misuse of a finalized canvas: re-raising
  one shared instance grew its traceback and kept every failed call's
  canvas alive.

| what | avg time before | avg time after | percent change |
| --- | --- | --- | --- |
| urwid.Pile.render (50 children, one child changed per frame) | 195 us | 203 us | +4% |
| urwid.CanvasCache.invalidate + store + fetch (50-child Pile) | 15.4 us | 21.8 us | +42% |
| urwid.CanvasCache.store (50-child Pile) | 14.6 us | 16.2 us | +11% |

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