ostree-ext: Add composefs-repo to ostree-commit synthesis

Add `container::composefs_import::import_from_composefs_repo`, which walks
a composefs `FileSystem` tree (already populated by a composefs-rs OCI
pull) and synthesizes an equivalent ostree commit, reflinking file content
into the ostree bare repo so the same disk blocks back both stores. This
is the inverse of libostree's ostree→EROFS path.

To support exporting such synthesized commits back out, `store.rs` gains
`export_commit` (export straight from a commit checksum rather than an
image ref) and `for_each_file_checksum` (walk a commit's regular-file
content checksums), `export_to_oci` is refactored to take a
`LayeredImageState` so both ref- and commit-based exports share it, and
`query_image_commit` learns to treat a composefs-synthesized merge commit
as its own base. A few existing metadata constants and helpers are
promoted to `pub`/`pub(crate)` for use by the new module.

This is library plumbing for bootc's unified storage work; it is unused by
any in-tree caller on its own and lands separately to keep that feature's
change set reviewable. Includes a standalone integration test
(`test_composefs_import_to_ostree`) that builds a small OCI image, imports
it through a composefs repo into ostree, and verifies the result.

Assisted-by: OpenCode (claude-opus-4-8)

Author: cgwalters