Final review found `pendingRetryFallbackModel` unreachable. `servingModel`
returns `undefined` only when the session has no model at all, and the
pending getter required one, so the badge term guarding on it could never
fire. Its case — a fallback armed before anything has served — is already
answered by `servingModel`'s bootstrap, which names the current model and
flags it as fallback-routed. Removed, the same duplicate-surface cleanup
that removed `retryFallbackModel`.
Attribution now anchors on the session id rather than the session file. An
unpersisted session has no file, so two `undefined`s compared equal and
stale attribution survived `/new` and branch switches there; every real
switch mints a new id, persisted or not.
The cooldown-expiry restore keeps `#fallbackRouted` when the stored primary
selector cannot be parsed. Nothing is restored on that path, so the session
is still running on the fallback and its remaining turns are still fallback
work; clearing the flag reported them as the configured primary.
`executor-prewalk`'s fake session predates this work and never set
`servingModel`, so the prewalk hand-off stopped advancing the reported
model once the executor began reading attribution from the session. It now
mirrors the hand-off the way the other executor fixtures do.
enieuwy
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