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fix(signals): an armed derived override shields an untracked read of an uninitialized node (A18(d), follow-up to #3651) (#3652)

A memo whose FIRST async landing rode its optimistic lane keeps
STATUS_UNINITIALIZED: the value sits in the derived-override slot until the
lane's commit promotes it (#3648, #3651). When the action then fails, A18
body-end supersession re-derives the memo from the truth on the plain
channel; in that window the node is pending, still uninitialized, and its
derived override is still armed and displayed.

An untracked read of it from ambient code threw NotReadyError: read()'s
`!c && STATUS_UNINITIALIZED` throw fired ahead of serve()'s override arm.
The same read of an INITIALIZED node re-deriving under its override already
returned the override. Maintainer ruling (A18 (d)): a pending node that
carries an override displays the override — "uninitialized must suspend"
(#3276, A19 exception 1) was written for a node with nothing to show. The
guard now yields to the override arm under #3651's predicate
(CONFIG_DERIVED_OVERRIDE && hasActiveOverride).

Unchanged: tracked lane readers keep the override (A17); tracked off-lane
readers suspend (#3651, overrideRead); settlePendingSource's invariant.

Spec: A17 lanes-stage mechanism index records #3651's ruling and this one;
RULES-INDEX regenerated.

Co-authored-by: Claude via Cursor <noreply@cursor.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

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