feat(emu): GUI debugger panel in Apple II emulator

[relmer]

Add a visual debugger panel to Casso65Emu, built on the debugger engine from #51.

Dependency: #51 (interactive debugger / monitor) provides the core engine — disassembler, breakpoints, watchpoints, step/continue. This issue covers the GUI front-end in the emulator.

Proposed features:

• Dockable or toggleable debugger panel (like the existing debug console, but richer)
• Disassembly view: current PC with surrounding instructions, highlight current line
• Register display: A, X, Y, SP, PC, flags (N/V/B/D/I/Z/C) — updated each step
• Memory hex dump: scrollable, editable, with ASCII column
• Breakpoint management: set/clear by clicking in disassembly, list view
• Watch expressions: user-defined memory/register watches
• Stack view: current stack contents from SP to \
• Step / Step-over / Continue / Run-to-cursor toolbar buttons
• Pause emulation to enter debug mode, resume to continue

Architecture:

• Debugger engine lives in Casso65Core (shared with CLI debugger from feat(cpu): interactive debugger / monitor #51)
• GUI front-end in Casso65Emu renders engine state into a child window or panel
• Debug mode pauses the CPU thread; step commands execute on the CPU thread

Priority: Medium — depends on #51 for the engine.