refactor(core): Split config, persist, api, and popup layers

[mhiro2]

Summary

• Split config into defaults, shared validation helpers, and concern-specific validation rules
• Split persist into cache, codec, and service layers, and replace placeholder persistence coverage with behavior-based specs
• Separate top-level API setup, registry, and dispatch wiring, and break popup creation into origin, buffer, and window concerns

Changes

• 36052c3 : refactor(config): split defaults and validation rules
  • Moved config defaults into a dedicated module and reduced peekstack.config to a thin facade
  • Split validation helpers and concern-specific rules so UI, picker, providers, and persist validation are organized by responsibility
• 813a8a1 : refactor(persist): split service layers and strengthen specs
  • Extracted persistence cache, codec, and service responsibilities to make save and restore flows easier to follow
  • Reworked persistence specs to assert real save/restore/list/delete/max_items behavior using actual popup data instead of placeholder checks
• 8eb3726 : refactor(api): split setup and dispatch wiring
  • Moved provider and picker registry, highlight setup, setup flow, and dispatch logic out of peekstack.init
  • Kept the public API intact while making peekstack.init focus on API wiring
• d5a3e27 : refactor(popup): isolate origin buffer and window setup
  • Split popup creation into origin capture, buffer preparation, and window opening concerns
  • Added focused tests for popup origin detection and buffer preparation so the new internal boundaries are directly covered