Implement additional Linux-compatible syscalls and syscall mappings (mmap, mprotect, fork/clone, wait4, umask, futex, etc.)

[pradosh-arduino]

Motivation

• Expand the FrostWing kernel's Linux syscall compatibility layer to support more userland expectations and to provide better argument validation and error reporting.
• Provide basic process/thread lifecycle primitives (fork/clone, wait4, set_tid_address, clear-child-tid) so userland programs that spawn and wait for children work as expected.
• Add basic anonymous memory mapping and protection validation to support common runtime behaviors from libc and interpreters.
• Improve futex and signal-related syscall stubs to return appropriate Linux errno codes rather than silently succeeding.

Description

• Added many Linux constants and flags to source/includes/syscalls.h including syscall numbers (CLONE, WAIT4), errno codes, PROT_*, and MAP_* definitions.
• Introduced source/kernel/C/syscalls-doc.dox documenting the Linux syscall wrappers and their kernel mappings.
• Implemented/updated syscall handlers in source/kernel/C/syscalls.c: sys_mmap, sys_mprotect, sys_munmap, sys_umask, sys_tgkill, sys_set_tid_address, sys_set_robust_list, sys_fork (used for both fork and clone), and sys_wait4, plus helpers to record exec context and track forked children; integrated with multitasking APIs for PID queries and spawning userland tasks.
• Hardened argument validation and Linux-compatible return codes across many syscalls (e.g. mmap alignment and flags, mprotect/munmap address/range checks, futex behavior and timeout handling), and switched some previously-no-op signal syscalls to return -LINUX_ENOSYS.
• Ensured syscall_handler clears clear_child_tid on exit and adjusted getpid/gettid to query multitasking_current_pid() when available.

Testing

• Built the kernel tree after changes and the build completed successfully with no compile errors.
• Booted the kernel in an automated QEMU-based smoke test and exercised userland syscall compatibility paths for execve, fork/clone + wait4, mmap/mprotect/munmap, umask, basic futex calls, and getrandom; the smoke tests completed without fatal failures.
• Ran automated unit-style checks for argument validation paths (alignment, mask checks, and returned errno values) and observed expected Linux-compatible error codes for invalid inputs.

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Codex Task