Stopping a turn or starting a new chat drops the DesignSession, but the
canvas indicators only cleared when the worker thread next touched its
channel and unwound — which lags seconds behind when the worker is
mid-LLM-call. The breathing borders lingered on a canvas the user had
already moved on from.
Give the host (which owns the turn lifecycle) the indicator epoch:
DesignSession::start mints it (clearing the prior turn at once) and the
session's Drop ends it immediately via end_if_epoch. The worker registers
under the same epoch via Orchestrator::with_indicator_epoch; headless and
test callers pass none and the concurrent path mints its own, so run()'s
signature and the DesignRequest construction sites are untouched.
end_if_epoch clears AND retires the epoch (vs clear_if_epoch which only
clears), so a worker still in its add_frame loop when the turn is stopped
can't re-populate the set under the now-stale epoch. It's epoch-scoped, so
a newer turn that already began is left untouched.
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