feat(podman): switch rootless storage driver to btrfs

fuse-overlayfs is dog-slow on `podman commit` (and noticeably slower
than native overlay/btrfs for layer extraction in general) because every
read/write round-trips through a FUSE daemon. The kernel overlay driver
does not support btrfs as a lowerdir, so on a btrfs root fs the choices
were:

  - fuse-overlayfs  (slow, but works)
  - btrfs           (native subvolume + CoW snapshot per layer; fast)

Switching graph drivers is destructive — the on-disk layout is
incompatible, so a one-time `podman system reset --force` is required.
A migration helper script lives at the repo root (gitignored,
chezmoiignored) that snapshots stateful containers, exports images and
volumes, runs the reset, and restores everything on the new driver.

Drops fuse-overlayfs from meta/base.txt — no longer needed and pulls
in libfuse3 transitively for nothing. (Flatpak still depends on it for
its own sandbox; pacman won't actually uninstall the binary while
flatpak is around — that's fine.)

VM (nix/vm.nix) is unaffected: it sets its own storage.conf inline
with driver=overlay since its rootfs is ext4.

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