Roll channel and node reminders across 6-hourly buckets

[TheBlueMatt]

Clients backdate everything we send them by a week and prune anything
they haven't heard about in two weeks, so a channel direction lives for
seven days past the header of the last snapshot that wrote it. Since we
never send value-identical re-broadcasts, an unchanging channel is only
written by a reminder. Reminders were previously included only in the
snapshot generated at 00:00 UTC every fifth day (or for scopes above 50
hours), i.e. in one three-hour window out of forty for any client
syncing at least every two days. Clients that missed that window, or
synced daily at a fixed hour outside it, lost every stable channel a
week after their last reminder, and channels can never be re-announced
to an incremental client afterwards. Even clients that caught every
window only kept a channel that had mutated once and then gone quiet if
a reminder happened to fall in the single day between the six-day
eligibility age and the seven-day client lifetime.

Instead, every channel and node is now assigned to one of 24 buckets
(a mixed hash of the SCID, or the node id's x coordinate), one bucket
becomes due every six hours, and a snapshot includes the reminders of
every bucket that became due within its (last_sync, reference] window.
Consecutive snapshots thus chain without gaps and any client that syncs
at least once every six days is reminded of every stable channel once
per six-day cycle, regardless of which snapshots it fetches. Clients
further behind receive all buckets at once, which subsumes the old
scope rule.

The channel-side six-day streak-age gate (and the window-function query
that computed it) is dropped: with a six-day rotation it recreated the
timing lottery for freshly mutated channels, and skipping recently
mutated channels saves few bytes since those mostly carry an
incremental update anyway. Node reminders keep their eligibility
semantics but only evaluate the 14-day announcement history for nodes
whose bucket is due.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

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