v0.20.0 — themes[] in heartbeat

Theme inventory now ships alongside plugins[] so the dashboard can render a per-site Themes section, detect outdated themes, and surface update/activate actions on themes.

Added

• Inventory\ThemeInventory — collects via wp_get_themes() + update_themes site transient
• Scheduler::buildPayload() emits the new themes[] key with: slug, name, version, active (compared against get_stylesheet()), parent (Template: header for child themes), update_available, new_version
• Standalone-theme self-reference defense: when WP reports Template: <self> for parent themes, the connector strips that to null so the dashboard catalog never stores a self-referencing parent row

Changed

• Heartbeat success log line now includes themes=N alongside plugins=N for parity in operator diagnostics

Wire contract

Purely additive. Pre-v0.20 dashboards ignore the new key. The matching dashboard release distinguishes "themes key absent" (pre-v0.20 connector) from "themes key present and empty" so this rollout never prunes an existing theme inventory.