`session.history` built its projection baseline in `historyStateFor` and only
then awaited `presenterScopeFor`, which is what ensures the recorded preset's
standing mount. The unit table is process-wide while the units themselves are
registered by preset rows, so on a first cold read the baseline was serialized
before `todos` (and every other preset-owned key) existed, and every later read
served a complete one. Until this branch, a web lane happened to mount a preset
in `beforeAll` for unrelated reasons, which hid the ordering.
The baseline is now a thunk the handler reads after the scope resolves. Also
covers the roster-less early return of the unjoined-agent warning, which the
per-file coverage gate reported uncovered.
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