cmux-next: debug.mouse middle clicks keep their window-local point

[teamleaderleo]

Summary

Fix forward for two cmux-next swift test failures on feat-cmux-next. Both came in with 76c37a6 (debug.mouse middle button):

• DebugMouseButtonTests.middleButtonEventsKeepTheWindowLocalPoint: locationInWindow is (20, -700), expected (20, 180).
• WorkspaceRowMiddleClickTests.middleButtonEventsAtTheRowCloseTheWorkspace: wrong point, so the row never sees the click and nothing closes.

Cause: debug.mouse makes a middle-button event by copying an NSEvent through CGEvent to set buttonNumber 2. AppKit maps the copy back through the window server's frame for the window. The tests use windows that are never shown, and for those the server's frame differs from the window's own. The offset is the screen height minus the window height: 880 for the 200-point test window, 480 for the row harness. Setting the copy's global point alone did not help (first CI run on this PR).

Fix: DebugMouse.keepingWindowPoint builds the copy once and measures the window-local point AppKit got. It moves the global point by the difference and builds it again, so the event lands at the intended point whatever frame the server holds. The row test lives in the Sidebar package, which can't reach DebugMouse, so it does the same inline.

QuitSessionsTests is a separate base red, fixed by #17605.

Testing

• Both tests fail on feat-cmux-next (for example the cmux-next swift test job on cmux-next: chat summary keeps Outputs, Subagents and Sources in place #17602 at 00792b8). This PR's cmux-next swift test run is the green check. Swift does not build or run locally here.

Changelog

none

🤖 Generated with Claude Code

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[github-actions]

Not verified: macOS jobs did not run: compile and app tests were skipped on 6cc3514940; ci-status is green without them.

CI passes on 6cc3514940 (run 37516996810 attempt 1).
macOS jobs did not run: compile and app tests were skipped: Fast static checks skipped (the macos job needs both to succeed). Re-run CI once they pass, before merging.

Written by scripts/ci/classify_failures.py (ci-failure-attribution.yml); signatures are its SIGNATURES table. A machine verdict is the runner's fault, not this PR's; yours means the failing file is one this PR changes, also red on main that main's latest full suite fails the same way.