architecture.md §7.1 stated five executors and their thread counts, and
no code named them. dr_ui::executors now does: the Executor enum with
each one's thread name and the count §7.1 gives with its reason, and
spawn, which starts a thread named <executor>:<role> and marks it with
the executor it belongs to. The counts are the stated budget, not yet a
bound: a job still gets a thread of its own when it starts.
run marks its own thread as the UI executor before it builds the
window. net_runtime::build now returns a NetRuntime whose block_on
asserts, in debug and test builds, that the caller is not that thread;
everything else derefs to the tokio runtime. The login, folder-list and
remote-folder workers built the same runtime by hand and now take it
from net_runtime, so their block_on is guarded too.
Tests: a block_on on a thread marked as the UI executor panics naming
the UI thread; the same call on a worker returns; a spawned thread
carries its name and executor.
dtourolle