Add a new `bootc loader-entries set-options-for-source` command that
manages kernel arguments from independent sources (e.g. TuneD, admin)
by tracking ownership via `x-options-source-<name>` extension keys in
BLS config files. This solves the problem of karg accumulation on bootc
systems with transient /etc, where tools like TuneD lose their state
files on reboot and cannot track which kargs they previously set.

The command stages a new deployment with the updated kargs and source
keys. The kargs diff is computed by removing the old source's args and
adding the new ones, preserving all untracked options. Source keys
survive the staging roundtrip via ostree's `bootconfig-extra`
serialization (ostree >= 2026.1, version check active).

Staged deployment handling:
- No staged deployment: stages based on the booted commit
- Staged deployment exists (e.g. from `bootc upgrade`): replaces it
  using the staged commit and origin, preserving pending upgrades while
  layering the source kargs change on top

Includes a multi-reboot TMT integration test covering: input validation,
kargs surviving staging roundtrip, source replacement, multiple sources
coexisting (including pre-reboot), source removal, idempotency, system
kargs preservation, empty --options vs no --options, and staged
deployment interaction with bootc switch.

Example usage:
  bootc loader-entries set-options-for-source --source tuned \
    --options "isolcpus=1-3 nohz_full=1-3"

Closes: #899
See: ostreedev/ostree#3570

Assisted-by: OpenCode (Claude Opus 4.6)
Signed-off-by: Joseph Marrero Corchado <jmarrero@redhat.com>