docs(cache): state the .publish-* gap as a ticket, not a present state

The contract block asserted that neither side reclaims .publish-new- 'today'.
True when written and about to stop being true: gitdan#30 tracks adding both
.publish-* prefixes to the arbiter's enumeration, and a sibling track is
landing it this round. A comment that dates itself against a merge in flight
is worse than no comment.

Rewords all four sites (cache-lib.sh, publish-snapshot.sh, and README's table
row and prose) to reference gitdan#30 and keep the mechanism that made the
shape worth catching — .publish-new- is tagged per job per run exactly as
.stage- is — rather than the arbiter's momentary contents. The rule itself is
unchanged; it is the durable part, and it is what found this.

Adds the counting check while there: the five names here and the prefixes
ci-cache-reclaim.sh enumerates are meant to be the same length, so a mismatch
is the cheapest signal that one side gained a shape without telling the other.

scripts/cache-lib.sh

@@ -55,12 +55,17 @@
 # is invisible: an unreclaimed staging tree is a full clone of a multi-GB
 # target dir on the one volume whose entire problem is disk.
 #
-# Of the five above, the arbiter enumerates `.stage-`, `.evicting-` and
-# `.reading-`. The two `.publish-*` names predate the contract and are not in
-# it, which is why `.publish-new-` — tagged per job per run exactly as
-# `.stage-` is — is a shape neither side reclaims today. Closing that is a
-# change over there, not here; the rule above is what stops the list growing
-# another one.
+# The two `.publish-*` names predate the contract and were outside it when this
+# block was written — the rule catching an uncovered shape on its first
+# application. `.publish-new-` is the one that mattered: it is tagged per job
+# per run exactly as `.stage-` is, so a publisher killed before the swap
+# strands a tree under a name no later run of that script matches, which is
+# precisely the shape only the arbiter can reach. Bringing both in is tracked
+# as daniel/gitdan#30.
+#
+# The two lists are meant to be the same length. A shape here without a prefix
+# there is one side having changed without telling the other, and it is
+# cheapest to notice by counting.
 #
 # The two staleness constants the arbiter mirrors are part of the same
 # contract, and that half has a direction to it — see CACHE_READ_STALE_SECONDS