Add --flat mode for to-filesystem installation

Implement an experimental "flat" install mode for `bootc install
to-filesystem`. When `--flat` is passed, the container rootfs is
copied directly to the target filesystem without ostree or composefs
layering. Post-install, the system boots as a regular (non-bootc)
Linux system; day-2 operations (upgrade, rollback) are unavailable.

This addresses the "step 1" approach described in #1668: getting bootc
to perform a clean traditional-style OS install from a container
image.

Changes:
- Add `FLAT_INSTALL_MARKER` constant (`etc/.bootc-flat`)
- Add `--flat` flag to `InstallTargetFilesystemOpts`
- Add `flat: bool` field to `RootSetup` (defaulting to false in all
  existing construction sites including baseline.rs)
- Branch in `install_to_filesystem_impl` to call `flat_install()`
  before the composefs/ostree paths
- Implement `flat_install()` which:
  1. Copies container rootfs via `cp --archive --one-file-system`
  2. Finds the kernel using the existing `kernel::find_kernel()` API
  3. Copies vmlinuz and initramfs to `/boot/vmlinuz-<ver>` etc.
  4. Regenerates initramfs via dracut (omitting ostree modules),
     with graceful degradation if dracut is absent in the target
  5. Creates a BLS entry at `boot/loader/entries/flat-<ver>.conf`
  6. Installs the bootloader via the existing bootupd/zipl paths
  7. Writes the flat install marker
- Add unit test for `create_flat_bls_entry` verifying the generated
  BLS config format

Only self-install mode (running inside the container) is supported;
`--source-imgref` is rejected with a clear error. UKI kernels are
not yet supported (rejected with a clear error).

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