Keep Canvas completion reminders until the pane is visible

[onevcat]

Summary

A Canvas pane can keep keyboard focus after panning moves its card completely outside the viewport. Such a pane must retain its unread Done state until it returns to view.

Continue the Canvas acknowledgement implementation from #783, with its original commit preserved. Require the focused terminal's bounds to intersect its AppKit visible rectangle, in addition to the original key/visible-window and actual-responder checks. Partial visibility is sufficient. Keep navigation, broadcast input, and worktree-scoped notification behavior unchanged.

The completion-read prerequisite #781 is merged. This branch includes that implementation, and the combined behavior was tested locally.

Fixes #779.

Validation

• Reproduced the bug against the original Acknowledge viewed Canvas panes after Agent Island navigation #783 implementation combined with Keep agent completions unread until their pane is viewed #781: moving the focused pane outside the viewport changed Working to Idle instead of Done.
• Confirmed that visibleRect can be nonempty but outside the terminal bounds; the predicate uses rectangle intersection.
• Final combined Xcode tests passed (83 test methods), including offscreen/returned panes, partial intersection, 0.5x/1x/2x ancestor scaling, zero area, logical-only focus, inactive windows, Blocked state, normal-mode viewing, Active Agents, and Island.
• make check passed, including 146 script tests.
• make build-app passed.
• git diff --check passed.
• Native AppKit tests use actual first-responder transitions and view geometry; no live-agent GUI session was manually exercised.

Original contribution

Thanks to @SunChJ (SamsonCJ) for the Canvas implementation in #780, resubmitted as #783. Original commit 3635ff56d1f001e70fc7422ab8666da8523b6883 is retained unchanged as an ancestor of this branch. This follow-up adds viewport coverage and combined behavior tests. Thanks to @onevtail for reporting the offscreen case.

Use a merge commit to retain the original contribution in repository history.