Autotune.SeamA test seam of the search: one value PER DOMAIN (the treatment the timing-call observers got, gh-ocannl-1263). A domain's searches call the value set in that domain; a search in another domain neither reaches nor disturbs it, so searches in different domains -- which take turns, Concurrent_search -- can each be observed and injected into on their own. A newly spawned domain starts with the seam's default. Unlike install_timing_call_observer, setting a seam REPLACES its value rather than chaining onto it: a caller that wants the enclosing value too reads it with get first and calls it.
val get : 'a t -> 'aThe calling domain's current value.
val set : 'a t -> 'a -> Base.unitReplaces the calling domain's value for the rest of the domain's life (or until set again); what a long-lived harness installs.
val with_value : 'a t -> 'a -> (Base.unit -> 'b) -> 'bwith_value seam v body runs body with the calling domain's value replaced by v, restoring the previous value on return or exception, so scopes nest.