A channel gains a typed assigned_bot (event-logged, snapshotted). ASSIGN
<#channel> <bot> / UNASSIGN <#channel> is founder-gated (or admin oper). The
engine reconcile now also diffs channel assignments against tracked bot
memberships: a live bot IJOINs its assigned channels and PARTs unassigned ones
(new ServiceJoin/ServicePart actions), at burst and after commands; a deleted
bot's memberships are dropped. Engine test for join-on-assign / part-on-unassign.
Adds BotServ as a new module crate (fedserv-botserv) and a persisted bot
registry: a typed Bot{nick,user,host,gecos} threaded through the event log
(new bots map folded in apply, snapshotted, Local scope). BOT ADD/DEL/LIST
gated on the typed Priv::Admin. BotServ is a default service (uid suffix
AAAAAD). Next slices: introduce bots on the network, ASSIGN to channels, and
in-channel fantasy commands. Data + oper-gated engine tests.
