Move platform activity probes off the hot path

[Today20092]

Parent

#45

What to build

Keep operating-system activity probes off the five-second tracking hot path. Preserve native Windows behavior while macOS and X11 publish cached activity samples from a bounded worker that exists only while Resolve runs.

Acceptance criteria

• Windows activity remains native and is available to each Tracking Observation without child processes.
• macOS and X11 activity sampling runs at most once every fifteen seconds while Resolve is present.
• Repeated five-second Tracking Observations read immutable cached activity and spawn zero child processes.
• Resolve absence stops activity sampling completely.
• Missing utilities, permission denial, malformed output, timeout, missing display access, and Wayland produce an explicit unavailable state.
• Unavailable activity pauses billable transitions and never assumes Active Work.
• No PyObjC, custom X11 binding, WMI, or new watcher dependency is added.

Working branch

feature/zero-idle-tracking

Blocked by

• Give tracking one cadence owner #46