Wait for an in-flight shadow store of (path, view), so the caller restores
the shadow's result instead of re-assembling the same slice. Returns null —
no await needed — when nothing is in flight. The returned promise resolves
when the shadow pass ends or SHADOW_STORE_MAX_WAIT_MS elapses, and
REJECTS with the signal's reason (an AbortError) when signal aborts first:
a superseded foreground pass must stop promptly, not sit out the shadow.
Wait for an in-flight shadow store of
(path, view), so the caller restores the shadow's result instead of re-assembling the same slice. Returns null — no await needed — when nothing is in flight. The returned promise resolves when the shadow pass ends or SHADOW_STORE_MAX_WAIT_MS elapses, and REJECTS with the signal's reason (an AbortError) whensignalaborts first: a superseded foreground pass must stop promptly, not sit out the shadow.