Refactor: factor shared debug-window scaffolding into a common DxDialog base

[relmer]

Summary

The two debug tool windows — InputDebugPanel and Disk2DebugPanel — independently re-implement a large amount of common window/dialog scaffolding. Factor the shared behavior into a common base class (proposed name DxDialog — see naming note) and derive both panels from it.

Both currently implement IChromedPanelContent and are hosted in a ChromedPanelWindow via composition (m_window). They duplicate, near-verbatim:

• Keyboard focus management — a parallel *FocusStop enum, an m_focusStops tab-order vector rebuilt on every focus change, plus matching switches in ApplyFocus/HandleKey and hand-written SetFocusToStop(...) calls in every mouse handler. (Tracked separately below.)
• ListView scrollbar mouse wiring — arrow/thumb/track hit-testing, thumb-drag begin/update/end, SetCapture/ReleaseCapture plumbing in OnLButtonDown/OnMouseMove/OnLButtonUp.
• Tooltip viewport clamping — SetViewportSize on layout recompute.
• Row layout / spacing constants — kRowVGap96, header gaps, etc.
• Sticky-tail autoscroll — keep newest row visible only when already scrolled to the bottom.
• Window title plumbing — SetTitle on the chrome titlebar.

Goal

Extract a common base (DxDialog) that owns the ChromedPanelWindow, focus ring, scrollbar/mouse routing for a hosted ListView, tooltip clamping, and title handling. The two debug panels become thin subclasses that only declare their own widgets and event semantics.

Sub-task: rework focus management

As part of (or alongside) this, replace the enum-and-three-switches focus model with a focus manager over a vector<IFocusable*> tab ring:

• Each focusable widget implements a small common interface (Blur(), HandleKey(), OnLButtonDown()/hit-test, and an IsFocusable() predicate for conditional stops like the joystick/paddle checkboxes).
• Focus becomes an index into the vector; Tab and clicks just move the index, and the manager blurs the outgoing widget and focuses the incoming one.
• Separate Dropdown::Blur() (lose keyboard focus) from closing the popup. They are currently the same call (SetFocused(false) also sets m_open = false), which caused the dropdown-commit bug worked around with a !IsOpen() guard. Decoupling these removes the workaround.

Naming note

DxDialog is a reasonable name, though these are non-modal chromed tool windows rather than modal dialogs — ChromedDialog, DxToolWindow, or DxPanelContent are alternatives. Pick whatever reads best against the existing ChromedPanelWindow / IChromedPanelContent vocabulary.

Not urgent

The current code is functional; this is a maintainability refactor. Land it on its own branch so the diff is reviewable in isolation.