Fix five real bugs found by actually installing and running the plugin

First live end-to-end install on real Unraid hardware (all 8 built
packages installed via upgradepkg, rc.podman started, containers
pulled/run/networked/port-mapped) — surfaced five genuine bugs no
amount of container-based CI testing could have caught, since none of
them exist inside the vbatts/slackware:15.0 build container:

1. rc.podman never created $PODMAN_LOG_DIR before redirecting the
   podman system service's output into it, so the service failed to
   even start ("No such file or directory"). Added it alongside the
   existing PODMAN_RUN_DIR mkdir.

2. config/storage.conf hardcoded a [storage] table, and
   podman-config.sh's `sync` step appended a second one at boot with
   the real graphroot/runroot — TOML forbids defining the same table
   twice. Removed the template's [storage] entirely; sync already
   generates the whole thing.

3. config/policy.json had a "_comment" pseudo-field for
   documentation, but containers/image's policy parser rejects any
   unknown top-level key outright. JSON has no comment syntax; moved
   the rationale into docs/ARCHITECTURE.md instead.

4. netavark >= 2.0 dropped its iptables firewall driver entirely
   (verified: passing "iptables" is flatly rejected) — nftables or
   firewalld are the only remaining options, and firewalld needs
   systemd/dbus, which Unraid has neither of. Set firewall_driver =
   "nftables" explicitly and documented that Unraid OS doesn't ship
   the `nft` binary this needs (a slackware64 nftables package works;
   not yet wired into the build/install pipeline — see follow-up).

5. Every container failed with "crun: pivot_root: Invalid argument".
   Root cause: Unraid's / is permanently the kernel's initial "rootfs"
   pseudo-filesystem (Unraid never pivots to a real one at boot — the
   whole OS runs from RAM), and pivot_root(2) unconditionally rejects
   that as the old root. This is not new: Docker/runc hits the exact
   same kernel restriction on this exact host and silently falls back
   to an MS_MOVE-based chroot; crun has no such fallback, only a
   --no-pivot flag with no config-file equivalent. Added
   plugin/sbin/crun-no-pivot.sh, a thin wrapper that scans crun's full
   argument list (podman puts global flags before the subcommand, so
   the subcommand isn't reliably $1) and injects --no-pivot right
   after create/run, and pointed containers.conf's crun runtime at it.
   Also fixed the podman.plg postinstall's chmod glob
   (`podman-*.sh` -> `*.sh`), which would have skipped this new
   non-podman-prefixed sbin script.

Verified end-to-end on the real host: pull, run, real network
connectivity (wget through the container's bridge), and a published
port actually serving HTTP (curl through -p 8099:80 to nginx) all
work. --no-pivot's security tradeoff (disabling one particular
container-escape mitigation) was explicitly discussed with and
approved by the user before committing, given it must be the default
for any container to start at all on this platform.

Follow-up not yet done: nftables (needed for #4) is not yet a
packages/ component in the reproducible build pipeline — it was only
installed manually on the test host for this verification run.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

config/storage.conf

@@ -7,14 +7,13 @@
 #
 # Full reference: https://github.com/containers/storage/blob/main/docs/containers-storage.conf.5.md
 
-[storage]
-driver = "overlay"
-# runroot and graphroot are set at runtime by rc.podman based on
-# /boot/config/plugins/podman/podman.cfg (configurable storage location),
-# not hardcoded here. TODO: document the exact substitution mechanism once
-# rc.podman is implemented.
-# graphroot = "/var/lib/containers/storage"
-# runroot = "/var/run/containers/storage"
+# The [storage] table itself (driver, graphroot, runroot) is intentionally
+# NOT defined here — podman-config.sh's `sync` command appends it in full
+# at sync time, since graphroot depends on STORAGE_PATH from
+# /boot/config/plugins/podman/podman.cfg (configurable storage location) and
+# can't be known statically. A second [storage] table here would be a TOML
+# duplicate-key error once sync appends its own — see podman-config.sh's
+# cmd_sync for the generated content.
 
 [storage.options]
 # TODO: overlay-specific mount options once the loopback filesystem