Since leaking annots to a remote `chart` actor probably isn't a thing we
want to do (often), add a removal/deletion handler block to the
`remote_annotate()` ctx which can be triggered using a `{rm_annot: aid}`
msg.
Augmnent the `AnnotCtl` with,
- `.remove() which sends said msg (from above) and returns a `bool`
indicating success.
- add an `.open_rect()` acm which does the `.add_rect()` / `.remove()`
calls underneath for use in scope oriented client usage.
- add a `._annot_stack: AsyncExitStack` which will always have any/all
non-`.open_rect()` calls to `.add_rect()` register removal on client
teardown, to avoid leaking annots when a client finally disconnects.
- comment out the `.modify()` meth idea for now.
- rename all `Xstream` var-tags to `Xipc` names.
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