* filer: add a placement overlay seam for the write path
New volumes take their disk type, replication, and data center from the
explicit request or the matched filer.conf rule. That leaves no way for a
feature to steer a whole collection onto a medium without an operator
writing an fs.configure rule by hand.
Add a generic PlacementOverlay hook on the filer: a func that maps a
collection to a placement override, installed by a factory the way the
plugin-worker handlers register. detectStorageOption consults it between
the explicit request value and the filer.conf rule, so it overrides the
rule but yields to a value the caller asked for.
The seam names no feature concepts, so it stays generic; a downstream
build registers the overlay it wants (e.g. a storage-class Landing tier).
Claude-Session: https://claude.ai/code/session_01Ks16jnt4S7gdDk8cheQ3xu
* filer: address review on the placement overlay seam
Honor ResolvePlacement's ok flag explicitly rather than relying on empty
values falling through the util.Nvl chain, and log at V(4) when the
overlay steers a collection. Document that RegisterPlacementOverlay is
init-only, so the unsynchronized read in NewFiler cannot race the write.
Claude-Session: https://claude.ai/code/session_01Ks16jnt4S7gdDk8cheQ3xu
