* ci: add Incus + btrfs (CoW) integration leg

The existing integration suite preseeds the Incus pool with driver: dir,
which is full-copy and exercises only the fallback half of the storage
advisory added in #7. This leg flips the entrypoint to a btrfs loop pool
and runs navarisd with --incus-strict-pool-cow=true so any regression in
the advisory or its wiring fails the run hard.

The base integration suite is unchanged: scripts/incus-entrypoint.sh
defaults INCUS_STORAGE_DRIVER to dir and only switches when explicitly
overridden. docker-compose.integration-incus-cow.yml is a parallel
standalone compose file matching the existing -firecracker / -mixed
pattern; .github/workflows/integration-incus-cow.yml mirrors
integration.yml across alpine/3.21 and debian/12.

Dockerfile.incus now installs btrfs-progs explicitly (the incus package
already pulls it transitively, but explicit avoids relying on a
transitive dep that could go away).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix(ci): incus btrfs source path must be the bare pool name

Incus rejects any source under /var/lib/incus that is not exactly
/var/lib/incus/storage-pools/<pool-name>. Our preseed had ".img" on the
end, which tripped the validator and aborted init. Drop the suffix —
Incus auto-creates the loop file at the bare path.

Reproduced from the failing alpine/3.21 leg of #9: "Failed to create
storage pool 'default': Only allowed source path under '/var/lib/incus'
is '/var/lib/incus/storage-pools/default'".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix(ci): provision incus btrfs pool ourselves before init

Letting Incus auto-create a loop file (source + size in the preseed)
fails in a Docker volume because the btrfs driver checks the *parent*
directory's filesystem first and rejects with "Provided path does not
reside on a btrfs filesystem (detected ext4)". The Docker volume
backing /var/lib/incus is on the runner's ext4 host, which we can't
change.

Workaround: do the loop+mkfs+mount ourselves in the entrypoint, then
hand Incus a ready-to-use btrfs mountpoint. Idempotent via stat -f
-c %T. The preseed becomes a plain "use this existing mount" with no
size hint, which Incus accepts cleanly.

Reproduced from #9 alpine/3.21 fail: "Failed to create storage pool
'default': Provided path does not reside on a btrfs filesystem
(detected ext4)".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix(ci): mount incus btrfs source outside /var/lib/incus

Two validators in Incus's btrfs driver fight over the same path:

  - "Only allowed source path under /var/lib/incus is
    /var/lib/incus/storage-pools/default" (validates source path)
  - "Storage pool directory ... already exists" (Incus wants to
    create that exact directory itself as the pool's internal layout)

We can satisfy either but not both at the same path. Move the mount
to /var/lib/navaris-incus-btrfs (sibling of /var/lib/incus, neither
validator applies) and point source there. Incus accepts the
existing btrfs path and lays out its pool internals inside it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>