refactor(stack): Split core stack internals into modules

[mhiro2]

Summary

• Split peekstack.core.stack into smaller internal modules for shared state, common helpers, stack operations, and event handling
• Reduce the original stack file to a thin facade while keeping the public stack API unchanged
• Preserve existing stack behavior and verify the refactor with formatting, lint, and test runs

Changes

• baa4972 : refactor(stack): split core stack internals
  • Reduced lua/peekstack/core/stack.lua to a facade and moved stack registries and popup lookup logic into a dedicated state module
  • Isolated stack lifecycle and visibility operations from window and buffer event handling so close, focus, restore, hide/show, and stale cleanup flows are easier to maintain
• 89b2b45 : test(persist): stabilize silent notification spec
  • Reworked the silent persistence spec to wait for async completion instead of assuming notification timing is already settled
  • Asserted that the silent save and missing-session restore paths do not emit their own messages, which avoids unrelated async notifications causing false negatives in CI