Background workload sampler: host checkpoint

The reserved-board theme trial now accepts --workload backgrounds with a private, explicit still/video selection and bounded duration. It verifies the same workload artifact used by the static theme trial, previews and activates the exact opaque background choice, samples the Sway plus persistent Rust shell cgroups, and uses the existing finally restoration path. The result records CPU microseconds, mean CPU cores, sampled process RSS and cgroup memory peaks, sample count and elapsed time. It records no process command lines or theme source paths. Incomplete RSS, wrong media kind, changed background ID, or failed activation is a failed trial, never a missing-value pass.

The video arm now requires a private 0600 wallpaper status record matching the selected generation and source-relative asset fingerprint, an unchanged decoder process, advancing decoded/submitted/callback counters and timestamps, and two different native captures. Missing or stale status fails closed and still invokes restoration. A host fake exercises the success schema; the installed Rust wallpaper client at this checkpoint still rejects video, so there is no board playback pass. An mpv app window is not a wallpaper substitute. The baseline/static resource schema is not an on-device or card-budget measurement. Changing native captures and Wayland callbacks do not establish panel presentation; the operator must review the private raw captures and retain the physical gate.

Focused host commands:

python3 tests/test_handheld_theme_trial.py
python3 tests/test_theme_background_metrics.py
python3 tests/test_theme_background_status.py
python3 -m py_compile tools/handheld-theme-trial.py tools/theme_background_metrics.py tools/theme_background_status.py

The fixture exercises pinned choices, a static resource arm, strict video failure, baseline restoration, bounded process counts and sampled cgroup math. Status fixtures exercise privacy, source identity, restart, stalled frames, and a simulated advancing video arm. No board, video presentation, real touch or performance result is claimed; theme tasks 4.3 and 5.4 remain open.