reach_rank tests: onDegreeSnapshotLoad written while a previous test's singleflight refresh still reads it (-race flake) #224

Problem

cd cmd/server && go test -race ./... can fail in TestReachRank_RefreshHonoursBackoffAfterQueuedFailure with "race detected during execution of test". This was found during the #222 review, which does not touch these files.

• The test writes the package variable onDegreeSnapshotLoad (reach_rank_test.go ~969).
• Meanwhile, a singleflight refresh goroutine left over from the previous test, TestReachRank_ExpiredSnapshotAlwaysRefreshes, reads it (reach_rank.go ~311).
• The pair passed 15/15 under -race in isolation, on both master and the fix(store): move content-hash migration writes to the ingestor and merge duplicates in memory (#215) #222 head. It is a timing flake in the test design, not a production bug.

Fix direction

• Have the earlier test wait for its in-flight refresh to finish before it returns, or
• make the hook an atomic or a per-server field instead of a package variable.

Acceptance

• go test -race -count=20 -run 'TestReachRank' ./cmd/server/ passes with no race reports.
• The full -race suite no longer reports a race in reach_rank.