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.

tests/array_mount_gate.bats

@@ -0,0 +1,96 @@
+#!/usr/bin/env bash
+# Seam: a container that binds the entropy array must refuse to start until the
+# thing it binds is actually mounted.
+#
+# Live 2026-10-31, Dankserver: the array's systemd mount STARTED eight seconds
+# before the media containers were created, but a rootless quadlet cannot order
+# itself against a SYSTEM mount unit — `RequiresMountsFor=/mnt/entropy` resolves
+# to nothing in the user manager and is simply inert. So podman resolved the bind
+# against the pre-mount directory (which held one stale subdirectory) and both
+# Jellyfin and Plex came up blind: their library root looked empty, Jellyfin
+# logged thousands of `Removing item, Type: "Season"`, and the array sat at 80 %
+# util doing nothing useful until the host went down. A blind bind is not a
+# cosmetic fault — it is a metadata-deleting machine.
+#
+# The measured fix is `ExecStartPre=<repo>/scripts/wait-for-mount.sh <source>`:
+# it WAITS for the mount and then refuses, so `Restart=always` carries the unit
+# across a slow mount. A `ConditionPathIsMountPoint=` was measured on the same
+# host and is the wrong tool: an unmet condition logs `skipped, unmet condition
+# check` with NRestarts=0 and never a start attempt — the unit stays dead until
+# a human starts it. That is fail-dead, not fail-closed.
+#
+# Tests assert only observable behavior of the gate (exit status and the name it
+# refuses by) and the structural law over the quadlets; never the internals.
+
+load helpers
+
+GATE="$REPO_ROOT/scripts/wait-for-mount.sh"
+
+@test "the gate accepts a path that really is a mountpoint" {
+    run "$GATE" -t 2 /proc
+    [ "$status" -eq 0 ] || { printf '%s\n' "$output" >&2; return 1; }
+}
+
+@test "a plain directory is refused, and refused BY NAME" {
+    dir="$BATS_TEST_TMPDIR/not-a-mount"
+    mkdir "$dir"
+    run "$GATE" -t 1 "$dir"
+    [ "$status" -ne 0 ] || { printf 'accepted a plain directory\n' >&2; return 1; }
+    [[ "$output" == *"$dir"* ]] || { printf 'refusal did not name the path: %s\n' "$output" >&2; return 1; }
+}
+
+@test "a path that does not exist is refused by name" {
+    run "$GATE" -t 1 "$BATS_TEST_TMPDIR/does/not/exist"
+    [ "$status" -ne 0 ]
+    [[ "$output" == *"$BATS_TEST_TMPDIR/does/not/exist"* ]]
+}
+
+@test "the gate holds the deadline instead of deciding on one look" {
+    # A single check that failed at the wrong instant is the boot race itself:
+    # the mount lands a second later and the unit is dead anyway. So the window
+    # must be spent waiting, and that is observable from outside as elapsed time.
+    dir="$BATS_TEST_TMPDIR/never"
+    mkdir "$dir"
+    start=$SECONDS
+    run "$GATE" -t 4 "$dir"
+    elapsed=$((SECONDS - start))
+    [ "$status" -ne 0 ]
+    [ "$elapsed" -ge 4 ] || { printf 'gave up after %ss, window was 4s\n' "$elapsed" >&2; return 1; }
+}
+
+@test "the gate accepts a mount that arrives during its window" {
+    dir="$BATS_TEST_TMPDIR/later"
+    mkdir "$dir"
+    if ! mount --bind /proc "$dir" 2>/dev/null; then
+        skip "creating a mount needs privileges this box does not grant"
+    fi
+    run "$GATE" -t 4 "$dir"
+    umount "$dir" 2>/dev/null || sudo -n umount "$dir" 2>/dev/null || true
+    [ "$status" -eq 0 ] || { printf 'gate did not wait for a mount that arrived: %s\n' "$output" >&2; return 1; }
+}
+
+@test "no unit without the gate binds the array or the metadata view" {
+    bad=""
+    while IFS= read -r unit; do
+        sources=$(sed -n 's/^Volume=\([^:]*\):.*/\1/p' "$unit" \
+            | grep -E '^(/mnt/entropy|/var/lib/media-meta/view)$' | sort -u)
+        [ -n "$sources" ] || continue
+        for src in $sources; do
+            grep -q "ExecStartPre=.*wait-for-mount.sh.* $src\b" "$unit" \
+                || bad="$bad$unit binds $src with no start gate"$'\n'
+        done
+    done < <(find "$REPO_ROOT"/dankserver "$REPO_ROOT"/dankclaw "$REPO_ROOT"/frankbook-pro \
+                 -path '*/quadlet/*.container' -not -path '*.AGENTTRASH*')
+    [ -z "$bad" ] || { printf 'blind binds:\n%s' "$bad" >&2; return 1; }
+}
+
+@test "no gated unit keeps the fail-dead ConditionPathIsMountPoint" {
+    bad=""
+    while IFS= read -r unit; do
+        grep -q "wait-for-mount.sh" "$unit" || continue
+        grep -q "ConditionPathIsMountPoint" "$unit" \
+            && bad="$bad$unit: a gate and an unmet-condition skip cannot coexist"$'\n'
+    done < <(find "$REPO_ROOT"/dankserver "$REPO_ROOT"/dankclaw "$REPO_ROOT"/frankbook-pro \
+                 -path '*/quadlet/*.container' -not -path '*.AGENTTRASH*')
+    [ -z "$bad" ] || { printf 'fail-dead conditions:\n%s' "$bad" >&2; return 1; }
+}