Multi-monitor screen ownership and drawer placement

[bigFin]

Niri-first reliability pass for multi-monitor setups. Known sub-points:

• Bar pinned/open state survives monitor unplug/replug (see Visibilities name-keyed maps and persisted JSON)
• Drawer/popout anchoring follows the correct screen when outputs change
• No leaked per-screen state after replug cycles (check screens/bars maps over time via stateStats)
• DPMS off/on keeps placement

Notes from recent work: Visibilities.load() no longer retains object-keyed entries that leaked across replugs. Still needs real-hardware verification.

[bigFin]

Verified live on dual HDMI (ASUS VS228 + LG TV) with a dev build:

• Baseline: shell up on both outputs, 17 windows / 8 workspaces tracked, state machine idle, queue 0.
• Disabled secondary via niri msg output HDMI-A-2 off: compositor reported Disabled, shell kept processing (27 events queued/processed across 15 batches, serials advancing, queue drained to 0). No errors in 6000+ log lines.
• Re-enabled: output back at preferred mode, 7 workspaces / 15 windows, queue 0, idle.
• Poll counters stayed at 0 throughout (nothing visible held refs).

Remaining: physical unplug/replug and DPMS cycles still want a run whenever convenient.