bootloader: Add bootc loader-entries set-options-for-source support

[jmarrero]

bootloader: Add bootc loader-entries set-options-for-source support
On bootc systems with transient /etc, the state file
/etc/tuned/bootcmdline is lost after each reboot. This causes
TuneD to lose track of which kernel arguments it previously
set, leading to kargs stacking up on every reboot cycle.

bootc loader-entries set-options-for-source solves this by
recording kargs ownership directly in the BLS config on /boot,
which persists across reboots regardless of /etc transience.
With --source tuned, bootc automatically removes all previous
kargs from the "tuned" source and replaces them with the new
set, eliminating the need for TuneD to maintain its own state
for diffing.

This commit adds bootc source tracking to the bootloader plugin:

1. Detect set-options-for-source availability by checking
   "bootc loader-entries set-options-for-source --help" exit
   code during plugin init.

2. When available, use "bootc loader-entries
   set-options-for-source --source tuned --options ..." for
   applying kargs and the same command without --options for
   clearing all TuneD-owned kargs on profile removal.

3. The bootc path is a top-level dispatch branch, checked
   before rpm-ostree. Fallback chain: bootc -> rpm-ostree
   legacy --delete/--append -> GRUB2. All existing code paths
   are unchanged.

4. Continue writing to /etc/tuned/bootcmdline as best-effort
   for diagnostics and backward compatibility, but do not
   depend on it for correctness when bootc source tracking
   is available.

5. Add 15 unit tests for the bootloader plugin covering:
   bootc detection (3), apply (3), removal (2), init flag
   derivation (2), and top-level dispatch (5).

Requires: bootc with loader-entries set-options-for-source
(bootc PR #2114) and ostree >= 2026.1
See: bootc-dev/bootc#899
See: bootc-dev/bootc#2114

Assisted-by: OpenCode (Claude Opus 4.6)