Moves terminal RPC, binary streaming, slot allocation, and exit-subscription
bookkeeping out of the 9.7K-line Session class and into a dedicated controller
with a small facade (dispatch, handleBinaryFrame, killTerminalForClose,
killTerminalsUnderPath, dispose, getMetrics). Session loses 7 fields and 22
methods (-651 LOC) and the cluster's invariants now live next to their state.
