feat(desktop): roaming pet (opt-in)

[OutThisLife]

Summary

Adds an opt-in "roam" mode for the desktop floating pet. Instead of sitting in one spot, the pet wanders the app like a little platformer character: it walks along surfaces, hops up onto the composer / profile rail, drops back down, and patrols the base of any open overlay — with proper directional walk/jump animations, paced to the sprite so it steps instead of gliding.

• Opt-in via Settings → Appearance → Pet → Roam (off by default). Pure client behavior, persisted in localStorage like the pet's drag position — never runs when the pet is popped out into the OS overlay, and pauses while the agent is working (only roams when at rest).
• Surface-aware wander (usePetRoam): re-measures walkable ledges from the live DOM each beat — the window floor, the composer (data-slot="composer-surface"), and the profile rail (data-slot="profile-rail") — then walks / hops / falls between them. Tracks the composer growing, the sidebar opening, window resizes, and falls back to the floor when a perch disappears (e.g. dropped mid-air after a drag).
• Stands on the status bar, not over it: the bottom "floor" rests on the status bar's top edge (data-slot="statusbar") when present.
• Follows overlays: when a full-screen route overlay (settings / profiles / cron / agents / command-center) is open, the pet's surface swaps to a single ledge at the overlay card's bottom edge so it patrols there; closing it restores the normal surfaces and the pet drops back down. The ledge is derived from OverlayView's shared inset (titlebar-height + padding) via the existing useRouteOverlayActive() signal — no element measuring.
• Directional animation: roamWalkRow() uses the sprite's running-left / running-right rows for the travel direction, falling back to the generic running row + CSS mirror for pets that lack them. Hops/falls use the jump pose.
• Grounded, sprite-paced walk: translation speed is derived from the sprite's animation loop + on-screen size (≈ one body-width per loop), so the legs match the motion; the pet also sinks a few px so its feet meet each surface instead of hovering.
• Cache/render-safe: movement mutates el.style imperatively each frame (no per-frame React re-render, mirroring the existing drag handler); pose + direction publish through small nanostores ($petMotion, $petRoamDir) the canvas already subscribes to. State commits back to React only when the pet settles.

Implementation notes

• $petState folds in a roam pose only when $petAtRest (agent idle), so a wander can never mask real agent activity.
• No new core/model-tool surface — desktop-renderer only, plus three data-slot markers.

Test plan

• tsc --noEmit, eslint, vitest (store/pet.test.ts covers at-rest gating + pose precedence) all green.
• Toggle Roam on: pet patrols the status bar, strolls across the composer, hops onto the profile rail, drops back down — left/right walk + jump anims, feet flush on each surface.
• Open an overlay (Settings/Profiles/…): the pet patrols the bottom of the overlay; close it → it drops back to the normal surfaces.
• Drag into mid-air → it falls promptly to the surface below.
• Roam off: pet stays put (no gravity), fully draggable.
• Start an agent turn while roaming → pet stops and reacts in place; resumes on idle.