Use one automatic desktop configuration across GNOME and Plasma

[Staphylococcus]

Parent: #89. MVP slice 3 of 3. Target: v1.8.0. Depends on #216 and #217. Coordinate the default path with #132.

Outcome

A user configures LG Buddy's behavior once and moves between GNOME Wayland and Plasma Wayland without choosing a backend, editing configuration, or repeating setup. Together with the other two MVP slices, this makes a prerelease suitable for validation with the reporter of #195.

Implementation boundary

• Fresh setup and normal reconfiguration use source discovery automatically. Present idle timeout, app inhibition and restore behavior; remove backend selection from these normal flows.
• Integrate Add KWin inhibition source with prebuilt plugins and local compilation fallback #233's KWin provisioning into the existing setup/update flow: verify a compatible prebuilt first, compile locally only if needed, and continue with the other available sources if neither produces a working bridge. Build dependencies are conditional on that fallback. Ordinary KWin-source absence does not block setup, migration or activity monitoring.
• Keep the automatic configuration independent of the current desktop. Discover available interfaces; do not persist a resolved desktop choice, source-set preset or session profile that pins the next login. Follow Compose independent activity and inhibition capabilities for GNOME and Plasma #216's independent activity/inhibition capabilities and source-supported push/pull modes. Activity tracking meets inhibition only through the Boolean blanking gate. The app-inhibition setting configures that subsystem's preference section; it does not add an engine-side preference branch.
• Preserve existing auto configurations. Give existing explicit gnome, wayland and swayidle configurations a documented compatibility path: keep their defined behavior until an explicit, recoverable transition to automatic integration. A failed or cancelled transition preserves the previous settings.
• Preserve existing CLI compatibility during the MVP with clear legacy diagnostics. Final public CLI retirement and the broader recovery experience belong to Complete automatic integration recovery and public settings compatibility #219. Compatibility affecting the MVP itself must already be tested before prerelease.
• Present activity capability diagnostics separately from Compose independent activity and inhibition capabilities for GNOME and Plasma #216's inhibition section/aggregate diagnostics: the available source set, its contributions/check results, pending work, preference overrides, inhibition-release delay and legacy overrides. Expected availability under supported circumstances and plugin-provisioning outcomes are separate diagnostic information. A missing optional source means reduced coverage and adds no uncertainty verdict or exceptional policy state. Reports describe the same evaluation; historical results do not grant later permission. Do not infer availability from silence.
• Coordinate with Remove swayidle from automatic fallback selection #132 so the normal source path neither probes nor requires swayidle. Retain that issue's existing ownership of default-path dependency removal and explicit-legacy coverage.

Acceptance

• Fresh setup completes without a backend question; behavior settings remain identical across the GNOME/Plasma round trip.
• Runtime discovery leaves the portable configuration unchanged and uses no desktop/session-profile selection. Concurrent SSH/console logins do not change the available graphical integration path.
• Existing auto and explicit backend values have tested upgrade behavior. Opting into automatic integration is a one-time transition, with no repeated migration per desktop switch.
• Current CLI usage is either preserved or receives its documented compatibility response; legacy overrides remain observable.
• Installed GUI, configuration and release-bundle tests cover the automatic path without swayidle installed, plus preserved explicit legacy usage.
• Add KWin inhibition source with prebuilt plugins and local compilation fallback #233's prebuilt success, local compilation fallback and absent-KWin outcome all complete setup with the same portable behavior configuration. Verify expected source availability for each test environment; missing inhibition support does not disable activity or other available sources.
• The three MVP slices pass the prerelease readiness checklist in Make desktop integration automatic #89, including playback before/after startup and basic diagnostics.

Release boundary

Completion enables the MVP prerelease checkpoint. It does not close #89 or require completion of #219, additional desktop adapters, #87 or #137. Publication follows the existing reviewed promotion and publication workflow. Reporter findings feed follow-up work before stable readiness is assessed.

[Staphylococcus]

Completed in #237, merged into dev after #132/#236.

Fresh setup and normal reconfiguration now use automatic source discovery. Explicit GNOME, Wayland and swayidle configurations retain their behavior until a confirmed transition, with cancellation and failure preserving the prior settings. The live report separates activity availability from inhibition checks, pending work and completed evaluation history. KWin provisioning retains prebuilt → conditional local build → ordinary source absence.

All 13 final CI jobs passed. The exact rebuilt bundle passed normal installation and GNOME 50.4 → Plasma 6.7.5 → GNOME with byte-identical configuration and an SSH session retained, plus native playback before/after monitor startup, overlap, release timing, gamepad restore, KWin loss/recovery, independent PowerDevil coverage and preference bypass. The complete lock and actual suspend/resume matrix passed on the preceding bundle before the final diagnostic-only history refinement. Installed GUI, legacy transitions/rollback, upgrades, no-swayidle and prebuilt/local/absent provisioning checks pass.

This completes the three-slice MVP implementation and validation checkpoint in #89. The artifact is CI-only; reviewed promotion/publication and the #195 reporter's application results remain separate. #89 stays open for publication, field validation and #219's remaining product work.