Fix health checks: Add cross-platform memory and load average check fallbacks

[Samarth1306w]

Description

Added cross-platform fallbacks for check_memory_usage() and check_load_average() to support environments without /proc/meminfo and /proc/loadavg files (e.g. macOS/Darwin, Windows).

Changes

• Modified check_memory_usage() to fall back to sysctl and vm_stat parsing on macOS/Darwin, and ctypes (kernel32.GlobalMemoryStatusEx) on Windows.
• Modified check_load_average() to fall back to os.getloadavg() where available (e.g., macOS/Darwin).
• Implemented unit tests in tools/test_health_check_fallback.py to assert memory and load check fallback functions by mocking missing /proc files and matching host output structures.
• Patched compilation scripts to map Android/Termux environments to Linux configurations.
• Included the generated diagnostic logs and build metadata under diagnostic/.

/claim #1