install/flat: Use composefs pipeline for rootfs

--flat install implementation will go through the composefs
repository. This gives several advantages:

- SELinux labeling is handled by composefs-rs (selabel), which applies
  labels from the image's file_contexts rather than inheriting the
  running container's labels.
- The kernel installation flow is reused from composefs-rs
  (get_boot_resources / UsrLibModulesVmlinuz).
- The composefs repo is preserved at /sysroot/composefs, making it
  easy to convert to immutable/bootc mode later. Users who don't want
  the metadata overhead can rm -rf /sysroot/composefs.
- write_to_path supports reflink copies on btrfs/XFS, sharing blocks
  with the composefs object store.
- --source-imgref now works with --flat (the in_host_mountns guard is
  removed since we pull via the image reference, not via cp /).

Signed-off-by: Eric Curtin <eric.curtin@docker.com>

Author: ericcurtin

ADOPTERS.md

@@ -7,11 +7,11 @@
 
 | Type | Name | Since | Website | Use-Case |
 |:-|:-|:-|:-|:-|
-Vendor | Red Hat | 2024 | https://redhat.com | Image Based Linux 
-Vendor | HeliumOS | 2024 | https://www.heliumos.org/ | An atomic desktop operating system for your devices
-Vendor | AlmaLinux (Atomic SIG) | 2025 | [atomic-desktop](https://github.com/AlmaLinux/atomic-desktop), [atomic-workstation](https://github.com/AlmaLinux/atomic-workstation) | Atomic AlmaLinux desktop respins
-Vendor | Caligra | 2025 | [workbench](https://caligra.com/workbench/) | An OS designed to accelerate knowledge work
-Vendor | CIQ | 2026 | https://ciq.com | Rocky Linux from CIQ (RLC) - Image Based Linux - Standard and Cloud variants
+| Vendor | Red Hat | 2024 | https://redhat.com | Image Based Linux |
+| Vendor | HeliumOS | 2024 | https://www.heliumos.org/ | An atomic desktop operating system for your devices |
+| Vendor | AlmaLinux (Atomic SIG) | 2025 | [atomic-desktop](https://github.com/AlmaLinux/atomic-desktop), [atomic-workstation](https://github.com/AlmaLinux/atomic-workstation) | Atomic AlmaLinux desktop respins |
+| Vendor | Caligra | 2025 | [workbench](https://caligra.com/workbench/) | An OS designed to accelerate knowledge work |
+| Vendor | CIQ | 2026 | https://ciq.com | Rocky Linux from CIQ (RLC) - Image Based Linux - Standard and Cloud variants |
 
 # bootc Adopters (indirect, via ostree)
 
@@ -24,13 +24,13 @@ is to be the successor to ostree, and it is our aim to seamlessly carry forward
 
 | Type | Name | Since | Website | Use-Case |
 |:-|:-|:-|:-|:-|
-| Vendor | Endless | 2014 | [link](https://www.endlessos.org/os) | A Completely Free, User-Friendly Operating System Packed with Educational Tools, Games, and More
-| Vendor | Red Hat | 2015 | [link](https://redhat.com) | Image Based Linux
-| Vendor | Apertis | 2020 | [link](https://apertis.org) | Collaborative OS platform for products
-| Vendor | Fedora Project | 2021 | [link](https://fedoraproject.org/atomic-desktops/) | An atomic desktop operating system aimed at good support for container-focused workflows
+| Vendor | Endless | 2014 | [link](https://www.endlessos.org/os) | A Completely Free, User-Friendly Operating System Packed with Educational Tools, Games, and More |
+| Vendor | Red Hat | 2015 | [link](https://redhat.com) | Image Based Linux |
+| Vendor | Apertis | 2020 | [link](https://apertis.org) | Collaborative OS platform for products |
+| Vendor | Fedora Project | 2021 | [link](https://fedoraproject.org/atomic-desktops/) | An atomic desktop operating system aimed at good support for container-focused workflows |
 | Vendor | Playtron GameOS | 2022 | [link](https://www.playtron.one/) | A video game console OS that has integration with the top PC game stores |
-| Vendor | Universal Blue   | 2022 | [link](https://universal-blue.org/) | The reliability of a Chromebook, but with the flexibility and power of a traditional Linux desktop
-| Vendor | Fyra Labs | 2024 | [link](https://fyralabs.com) | Bootc powers an experimental variant of Ultramarine Linux
+| Vendor | Universal Blue | 2022 | [link](https://universal-blue.org/) | The reliability of a Chromebook, but with the flexibility and power of a traditional Linux desktop |
+| Vendor | Fyra Labs | 2024 | [link](https://fyralabs.com) | Bootc powers an experimental variant of Ultramarine Linux |
 
 ### Adopter Types