* shell: warn when fs.mergeVolumes source holds only orphan needles
fs.mergeVolumes traverses filer entries, so a source volume whose
needles are all orphans — filer entries lost to a crashed write or a
wiped filer store — produces only the plan header and exits 0: no move,
no skip, no error. Operators read that as a successful merge while the
real cleanup (volume.fsck) never runs, and dat>idx volumes keep coming
back read-only after restarts.
Count the source-volume needles seen during traversal and, when a plan
source was never seen but its index still reports needles, print a
warning pointing at volume.fsck. Dry-run warns too.
* shell: make needle counting concurrency-safe and count manifest sub-chunks
TraverseBfs runs its callbacks from five workers, so the plain
needlesSeen map raced between source-heavy merges (fatal concurrent
map writes). All increments now funnel through a mutex-guarded
recordSeen closure.
Manifest sub-chunks that live on planned source volumes are now
recorded too — rewriteManifestChunk visits them (including dry-run
and capacity-skipped ones) but previously never marked their source,
which produced false 'orphan needles' warnings for sources whose
chunks were all reached through manifests.
* shell: extract sourceNeedleCounter so the concurrency test covers the production path
The orphan-warning recording was a closure local to Do, so
TestWarnUnreferencedSources_ConcurrentRecording could only exercise a
test-local copy of it — a regression in the production mutex would pass
the test. Lift the map and mutex into a sourceNeedleCounter type with
record/count methods and use it from Do and the test, so the -race test
now drives the actual recording path. Trim the verbose comments added
with the warning while here.
---------
Co-authored-by: Chris Lu <chris.lu@gmail.com>
