[Feature] Window - needs DPI aware size, need to support multiple windows

[codemonkeychris]

Problem

Just a missing component... need Window. Icon, Title, Size, etc... size should be DIPs/DPI aware. 'nuff said.

Proposed solution

make Window

Alternatives considered

No response

Additional context

No response

Confirmation

• I have searched existing issues and specs for prior discussion of this idea.

[codemonkeychris]

Spec landed in #191 and merged to main: docs/specs/036-window-design.md.

It covers the items in this issue and goes broader:

• DPI-aware sizing — all WindowSpec dimensions are DIPs; GetDpiForWindow + WM_DPICHANGED / WM_GETMINMAXINFO hooks; UseDpi() hook for re-renders on monitor crossings (§5)
• Icon, Title, Size, min/max, presenter, chrome — full WindowSpec record (§4.1)
• Multiple top-level windows — ReactorApp.Run(Action<ReactorAppContext>) + ctx.OpenWindow(...) + UseOpenWindow(key, spec, factory) hook (§4.3, §7)
• Persistence — PersistenceId with packaged + unpackaged storage adapters (§8)
• Full Windows shell surface — taskbar progress, overlay icon, jump list, system tray (with reconciled flyout), thumbnail toolbar, launch activation (§11)
• Migration — ReactorApp.Run<TRoot>(...) keeps its signature; width/height now mean DIPs (the historically intended behavior) (§12)

Implementation is split into 8 phases; phases 4–6 are gated by 1–3, and phases 7–8 (shell integration) can ship in parallel with 4–6 once ReactorWindow exists. Tracking the implementation work separately as it lands.

[codemonkeychris]

#215