fix(fleet): refuse a container start until the mount it binds has answered

Dankserver rebooted with the array's mount and the media containers racing. A
rootless quadlet cannot order itself against the system's mnt-entropy.mount, so
`RequiresMountsFor=/mnt/entropy` is inert in the user manager: podman resolved
the bind against the pre-mount directory, Jellyfin and Plex came up blind, and
Jellyfin read an empty library and began `Removing item, Type: "Season"` by the
thousand. Copyparty had no guard at all.

`scripts/wait-for-mount.sh` is the gate: it waits for every named mountpoint,
then refuses by name. `ConditionPathIsMountPoint=` is NOT the fix — measured on
the host, an unmet condition logs `skipped, unmet condition check` with
NRestarts=0 and no start attempt ever follows, which is fail-dead, not
fail-closed. Jellyfin and Plex swap the condition for the gate; Copyparty gets
one; the law is recorded in AGENTS.md and the view page.

Verified on the host: the 5.7.0 quadlet generator accepts ExecStartPre in
[Service] (exit 0, all three units generated, gate present), the gate waits for
a mount that appears mid-window and proceeds, and it refuses a plain directory
after its window naming the path.

tests/media_overlay.bats pinned the old mechanism by name; its intent — both
media servers refuse to start without the view — now rides the gate.

dankserver/copyparty/quadlet/copyparty.container

@@ -36,6 +36,9 @@ Volume=/home/dank/Containers/copyparty/cfg:/cfg
 PublishPort=3923:3923
 
 [Service]
+# /w is the array: serving the pre-mount directory instead of the volume shows an
+# empty share and nothing complains, so the mount has to answer first.
+ExecStartPre=/home/dank/serverAppConfigs/scripts/wait-for-mount.sh /mnt/entropy
 EnvironmentFile=/home/dank/Containers/copyparty/env
 Restart=always
 RestartSec=5