style(prune-cache): rewrap the settle-window comment; drop an overclaim

Review nits on #2, both cosmetic.

The sentence added last round left its paragraph at 127 characters in a file
that otherwise wraps comments at 78-79 — the rewrap after the insert simply
did not happen. `awk 'length($0)>80'` over both files now reports one comment
line, the pre-existing 93-character usage string at :4.

Scenario 14's header called its fixture "the only shape that can tell the two
timestamps apart". The property it needs is old mtime with fresh ctime; an
old directory renamed a moment ago is the production instance of that, not
the only construction of it. "Production's shape" was already carrying the
argument.

No behaviour change and no assertion change.

scripts/prune-cache.sh

@@ -92,9 +92,10 @@ STALE_LOCK_SECONDS="${STALE_LOCK_SECONDS:-7200}"
 # and it separates them without having to identify the pass that created it.
 # The `$$` in an aside's name was that pass's PID inside its own job
 # container, so testing it with `kill -0` from a different one is not
-# unreliable, it is meaningless — and PIDs recycle besides. Three orders of magnitude of headroom over the operation it covers,
-# and short enough that a genuine leftover is reclaimed by the next run rather
-# than lingering while the volume is under pressure.
+# unreliable, it is meaningless — and PIDs recycle besides. Three orders of
+# magnitude of headroom over the operation it covers, and short enough that a
+# genuine leftover is reclaimed by the next run rather than lingering while
+# the volume is under pressure.
 EVICTION_ASIDE_SETTLE_SECONDS="${EVICTION_ASIDE_SETTLE_SECONDS:-60}"
 
 declare -A protected_ns=()