feat: add btop system monitor tab (F3) to kernel terminal manager

[ryanbreen]

Add a third tab to the kernel terminal manager that displays live system monitoring data: process list, memory usage, uptime, and tracing counters. The monitor auto-refreshes every ~1 second using a tick-based check in the render thread.

Key changes:

• Terminal manager extended from 2 tabs to 3 (Shell/Logs/Monitor)
• F3 key and mouse click support for Monitor tab switching
• render_btop_view() collects process data under try_lock, releases before formatting to avoid single-CPU ARM64 deadlocks
• Render thread checks get_ticks() directly for refresh timing, avoiding feature-flag dependencies on timer ISR
• Fix fork state transition: both ARM64 and x86_64 fork paths now call set_main_thread() to properly transition Creating → Ready
• Revert init.rs to spawn bsh directly (not bwm)
• Revert sys_fbmmap to always use left-pane-only mapping
• Add lowercase a-z and symbol support to bitmap font
• Add kernel log ring buffer, procfs additions, cpu_ticks field
• Add standalone userspace btop.rs and bwm.rs programs
• Add PTY infrastructure and libbreenix graphics wrappers