v0.1.30-pre.76

preview: nothing a person can see changes in this one, and that is provable rather than
claimed: no host passes the store context at all, so in every build a person can install the
four screen lines this changes already dispatched to the same object they do now. Underneath,
ui's four mark-read sites go through the session's own seam instead of naming the global
dispatcher, with a build check refusing the next screen that picks a pool of its own and a
test that watches which side of the seam a screen's write arrives on.

No migration: schema stays at 22 as at pre.75, so an account this build opens still needs
pre.61 or newer. Going back below that drops you at the account picker as though the account
were gone.

Landed from pr-bunk, pull request 0a6d467d at tip f713466a, merge ec7adc0c. Fixes #0b6ec6d9.