Follow-up to #297: pin a merge that moves first_seen and changes the path; stale Hash in distance index after a no-move rehash #303

From the review of #297 (#288, merged).

1. Test gap. No test covers a merge that moves first_seen and changes the path. The XOR mutant pathChanged != moved survives all current tests, and it would leave that survivor's distance records stale. Add a test.
2. Pre-existing. A rehash where neither the path nor first_seen moves leaves the old Hash in distHops/distPaths until the next full build. Decide whether to refresh the hash too, or document why it is harmless.
3. Wording in fix(server): refresh distance index when a merge moves FirstSeen (#288) #297. Before the lazy distance build has run, updateDistanceIndexForTxs still appends records for the listed survivors into the unbuilt index. Check that this is harmless, or skip it while the index is unbuilt.