test(prune-cache): build scenario 14's aside the way the pass builds one

Review finding on #2. Swapping the settle window's `stat -c %Z` for `%Y` —
the exact substitution the comment beside it calls the wrong signal — left
the suite green at 37/37. The scenario built its aside with `mkdir`, so the
fixture's mtime was also ~now and the two timestamps agreed; a fixture whose
clocks agree cannot tell them apart.

Every real aside is the opposite shape: a cache last written days ago,
renamed a moment ago. Under `%Y` the window would never fire for one, the
sweeper would silently return to reclaiming asides another pass is still
deciding about, and nothing would say so — the mechanism guarded by a comment
again, which is what the previous finding was about.

So the fixture is now built the way the pass builds one: an old directory
`mv`d into the aside name. `%Z` -> `%Y` now fails scenario 14, as does
deleting the guard outright.

Also sharpens why the PID alternative was rejected: the `$$` in an aside's
name was that pass's PID inside its own job container, so testing it from
another one is not unreliable, it is meaningless.

No change to prune-cache.sh's behaviour; the assertions are untouched.

scripts/prune-cache.sh

@@ -89,9 +89,10 @@ STALE_LOCK_SECONDS="${STALE_LOCK_SECONDS:-7200}"
 # An aside directory is in flight for one rename plus one marker glob —
 # milliseconds. Anything older belongs to a pass that died between the two, so
 # an age is what separates "another pass is mid-eviction" from "a leftover",
-# and it separates them without having to identify the pass that created it: a
-# PID is meaningless across the job containers these passes run in, and
-# recycles. Three orders of magnitude of headroom over the operation it covers,
+# 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.
 EVICTION_ASIDE_SETTLE_SECONDS="${EVICTION_ASIDE_SETTLE_SECONDS:-60}"