fix(mac): keep right-click status-item menu open and stable

[hyknerf]

Summary

• Fixes the menubar right-click context menu on macOS 27 (and keeps ≤26 working): menu no longer flashes closed, and no longer jumps/scrolls the top "Today · …" row under a chevron on mouse move.
• Flash: global monitor presented on rightMouseDown; matching rightMouseUp dismissed the menu. Monitor is now rightMouseUp.
• Jump: manual NSMenu.popUp(at:in:) tracked poorly while the cursor sat on the status item. Open via statusItem.menu + performClick instead; clear the menu in menuDidClose so left-click still opens the popover.
• Closes macOS menubar: right-click context menu flashes and/or scrolls away on mouse move #802.

Context

#472 restored right-click on macOS 27 with a global monitor + popUp, but presentation on mouse-down caused the flash. #509 tried height + 6 anchoring to avoid scroll; NSStatusBarButton is flipped and manual popUp still engaged scroll mode on mouse move.

#472 avoided statusItem.menu + performClick because the right-mouse action never runs on 27. We still open from the global monitor (or legacy action on ≤26); once statusItem.menu is set, performClick only means "open the attached menu" and works on 27.

Test plan

• ./mac/Scripts/package-app.sh dev succeeds (Xcode 26.6 / Swift 6.3)
• Built binary has global-monitor mask 0x10 (rightMouseUp) and references performClick
• Installed dev build on macOS 27 beta; right-click opens menu (flash path fixed in earlier binary probe)
• Please verify on macOS 27: right-click stays open; mouse move does not show ^ / scroll away Today row; Settings / Quit work
• Please verify on macOS 27: after dismissing the menu, left-click still opens the popover
• Smoke on macOS ≤26 if available: right-click presents exactly once (0.3s debounce); no flash; no jump

Notes

• Single file: mac/Sources/CodeBurnMenubar/CodeBurnApp.swift
• No CLI / TS changes — npm test N/A for this diff
• ≤26 still relies on debounce if both legacy action and global monitor fire (same as fix(mac): restore right-click status-item menu on macOS 27 #472)

[hyknerf]

Added unit tests for the extractable policy behind this fix:

• StatusItemContextMenuPolicy — event mask (rightMouseUp), presentation (.statusItemMenu not .manualPopUp), debounce gate
• StatusItemContextMenuPolicyTests — 7 cases, all green via swift test --filter StatusItemContextMenuPolicyTests

AppKit open/track/dismiss still needs a manual smoke (no good headless seam for NSStatusItem + real menu tracking). The tests lock the decisions that caused the regressions so they can’t silently flip back.