Fix: clear the discrete observation-buffer slot when an emitter is unsubscribed

[docxology]

Summary

When Rocket.unsubscribe!(emitter, receiver) severs an emitter from a discrete receiver, the receiver's
Observations.buffer still holds the emitter's slot (nothing after a clear, or a stale Observation). The
discrete forwarder (src/markovblanket.jl:73-80) requires all buffer slots to be filled before emitting, so
the removed emitter's dead slot blocks all future observations — the environment silently stops emitting to the
surviving agents.

Change

src/abstractentity.jl — in Rocket.unsubscribe!(emitter::AbstractEntity, receiver::AbstractEntity), when the
receiver is discrete, delete the emitter from the receiver's observation buffer:

function Rocket.unsubscribe!(emitter::AbstractEntity, receiver::AbstractEntity)
    Rocket.unsubscribe!(sensors(receiver)[emitter])
    delete!(sensors(receiver), emitter)
    delete!(actuators(emitter), receiver)
    # A discrete receiver buffers one observation per emitter; drop the removed
    # emitter's slot so the "wait for all emitters" check can still complete.
    if receiver isa AbstractEntity{T,DiscreteEntity} where {T}
        delete!(observations(receiver).buffer, emitter)
    end
end

How to verify

Add/run the discrete unsubscribe regression:

env = RxEnvironment(MockEnvironment(); is_discrete=true)
a1 = add!(env, MockEntity()); a2 = add!(env, MockEntity())
obs = keep(Any); subscribe_to_observations!(a1, obs)
send!(env, a1, 0.0); send!(env, a2, 0.0)   # obs.length == 1
unsubscribe!(a2, env)
send!(env, a1, 0.0)                          # obs.length == 2  (was stuck at 1)

Run julia --project=. -e 'using Pkg; Pkg.test()' — discrete entity tests remain green.

Notes

The continuous path is unaffected. This is a focused one-line-class fix; a follow-up could also add a
clear_buffer!-style reset on unsubscribe for robustness.