Draft: Add Powerwall live outage activity support

[CCOSTAN]

Summary

• add reusable script.notify_live_activity helper for tagged live activity/live update notifications
• add Powerwall outage live tracking using binary_sensor.powerwall_grid_status.last_changed as the outage anchor
• start/update the tagged outage activity for parents while off-grid and clear it after the grid is stable for 2 minutes
• update script/package READMEs to document the new helper and Powerwall behavior

Validation

• Home Assistant YAML DRY verifier passed for the touched YAML
• python -m homeassistant --script check_config --config /config passed inside the running home-assistant container

Notes

• Draft on purpose so this can stay parked until the upstream companion-app live activity support is merged
• Related issue: Add automations that push Live Activity notifications for critical workflows #1598
• Intended upstream dependency to watch: Adds iOS Live Activities support home-assistant/iOS#4444