Follow-up to #67. The runtime window contract is implemented and tested across macOS, Linux, and Windows. Page-originated replies and window lifecycle events route to the correct window and handle owner. Application-wide asynchronous sources still intentionally deliver to the primary page; opening a child must not silently redirect them.
Current sources are listed in docs/architecture/multi-window-ownership.md: theme changes, screen configuration, deep links, location updates, in-app purchase updates, local-server requests, screen-sharing changes, and tray/application-menu actions. Touch Bar ownership and advanced window-control parity are separate concerns.
Decide and document the delivery policy for each source: keep primary-only, broadcast to live pages, or track subscribers and their owning webviews. For subscriber-owned sources, define behavior across navigation, close, destroy, and orphan adoption; do not route events into a stale page. Preserve existing primary-page behavior until an explicit policy and migration path are approved.
Acceptance: document the per-source policy and compatibility impact; implement the selected routing and cleanup; add multi-window tests that prove the intended recipient receives events and unrelated pages do not; cover installed-app behavior where a real native source is required. This issue does not reopen the completed window lifecycle and parent-only modal contract in #67.