docs(nwsync): verify is what tells you which keys to purge

Running the repair for sow-tools#88 disproved the advice #90 had just
landed. "Purge the zone, then believe verify" assumed the stale set was
unknowable. It is not: verify reads the edge, so a run straight after a
repair names every key the edge is still serving stale — a survey, not a
verdict. Purge those, re-run, and the second run is the verdict.

The measured numbers are the argument. The repair rewrote 2,603 blobs at
the origin; 8 were stale at the edge, all of them ones a failed player
sync had pulled ninety minutes earlier. The edge only caches what someone
fetched, so purging the whole zone would have cooled 69,169 objects to
fix 8.

Refs #88, #89, #75.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/nwsync/run.go

@@ -67,8 +67,9 @@ check on a published blob upstream of a player's client.
 --verify makes emit hash what it would otherwise skip. emit normally treats a
 blob's presence as proof of its contents, so without this an object written
 truncated, or written by an emitter since found broken, is skipped forever.
---verify repairs the storage zone, while verify reads the edge in front of it,
-so purge the pull zone after a repair or verify answers differently per PoP.
+--verify repairs the storage zone, while verify reads the edge in front of it.
+So a verify run right after a repair is a survey, not a verdict: it names the
+keys the edge still serves stale. Purge those, then run it again.
 
 --out DIR writes to a local repository tree instead of uploading, which is the
 conformance path against upstream nwn_nwsync_write. Without it, the zone comes