feat(mm): implemented on-demand paging support for lazy stack growth (user faults only)

[FlareCoding]

Note

High Risk
Changes trap handlers, address-space mapping, stack layout, and uaccess paths in privileged kernel code; incorrect fault handling or lazy mapping could crash tasks or leak memory.

Overview
Adds user-only on-demand paging for lazy stack growth: x86 and AArch64 trap handlers translate fault details into mm::PF_FLAG_* and call mm::handle_user_pf before delivering SIGSEGV. The handler only resolves not-present faults on VMA_FLAG_STACK VMAs (allocates, zeros, maps a page; concurrent faults on the same page are treated as success).

MM_MAP_LAZY skips eager physical mapping in mm_context_map_anonymous; USER_STACK_MAX_PAGES (8 MiB) defines the stack reservation. create_user_task maps a large lazy stack VMA plus an eager top window for argv/envp setup. validate_user_range pre-faults missing lazy pages so kernel copy_*_user does not fault on unmapped PTEs.

mm_context and mapping lifecycle APIs move from vma.cpp into mm/mm.cpp and mm/mm.h; VMA helpers stay in vma with shared helpers exported. Call sites switch to #include "mm/mm.h".

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