composefs/boot: Clean up BLS sort-key and filename ordering

Improve boot entry ordering to work correctly across both Grub and systemd-boot
bootloaders, which have fundamentally different sorting behaviors.

Background:
Grub does not read BLS fields - it parses the filename as an RPM package name
using split_package_string(). The parsing splits on `-` from right to left:
1. Strip .conf suffix
2. Find LAST `-` → extract "release" field
3. Find SECOND-TO-LAST `-` → extract "version" field
4. Remainder → "name" field
5. Sort by (name, version, release) in DESCENDING order

See: ostreedev/ostree#2961

Changes:
- Add comprehensive module documentation explaining bootloader sorting behaviors
- Parse os-release to extract ID field (e.g., "fedora", "rhel")
- Filename format: `bootc_{os_id}-{version}-{priority}.conf`
  * Replace `-` with `_` in os_id to prevent Grub mis-parsing
  * Priority in release position for Grub compatibility
  * Primary: `bootc_fedora-41.20251125.0-1.conf`
  * Secondary: `bootc_fedora-41.20251124.0-0.conf`
- Sort-key format for systemd-boot:
  * Primary: `bootc-{os_id}-0` (sorts first)
  * Secondary: `bootc-{os_id}-1` (sorts second)
- Update rollback logic for new filename format
- Add comprehensive unit tests

Boot entry ordering after upgrade (both bootloaders):
1. Primary: New/upgraded deployment (default boot target)
2. Secondary: Currently booted deployment (rollback option)

Sorting behavior:
- Grub: Descending by (name, version, release) from filename parsing
- Systemd-boot: Ascending by sort-key field, filename mostly irrelevant

Fixes: #1777
Related: ostreedev/ostree#2961

Signed-off-by: Colin Walters <walters@verbum.org>
Assisted-by: Claude Code (Sonnet 4.5)
Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters