feat(stack): Add stack-wide popup visibility toggle with safe restore

[mhiro2]

Summary

• Added :PeekstackToggleVisibility to temporarily hide/show all popups in the current stack.
• Introduced a hidden stack state where popup models remain in memory while popup windows are closed.
• Hardened hide/show behavior by suppressing window/buffer close handlers during intentional hide operations.
• Expanded tests to run with events.setup() enabled and verified no history entries are leaked during hide.

Changes

• e8ae5bf : feat(stack): add popup visibility toggle with safe restore
  • Implemented stack visibility toggle flow (hide by closing popup windows, show by reopening from stored
    models).
  • Added a suppress_win_events guard to prevent WinClosed/BufWipeout handlers from removing
    intentionally hidden popups.
  • Added/updated visibility-toggle tests, including coverage for event-enabled execution and history non-
    leak behavior.