fix(arm64): achieve full Copy-on-Write parity with x86-64

[ryanbreen]

Summary

This PR fixes multiple ARM64 Copy-on-Write implementation issues to achieve complete parity with x86-64. All 6 CoW tests now pass on ARM64.

Fixes Implemented

• CoW statistics counters: Added to ARM64 page fault handler to track faults, copies, and optimizations
• TLB flush on TTBR0 switch: Added tlbi vmalle1is in context_switch.rs and syscall_entry.rs - ARM64 requires explicit TLB invalidation unlike x86-64's automatic CR3 behavior
• Fork stack copy via HHDM: Fixed to use physical addresses through HHDM instead of virtual addresses (TTBR0 still points to parent during copy, causing corruption)
• cleanup_cow_frames: Implemented for ARM64 to properly decrement reference counts when process exits
• sbrk check: Fixed in libbreenix to use >= instead of == for page alignment
• cow_oom_test design: Fixed to enable OOM after fork (fork requires frame allocation for page tables)
• Exception handler SIGSEGV: Fixed to terminate userspace processes with SIGSEGV on unhandled data aborts instead of hanging

Test Results (ARM64)

Test	Result
cow_readonly_test	✅ PASS
cow_signal_test	✅ PASS
cow_cleanup_test	✅ PASS
cow_sole_owner_test	✅ PASS
cow_stress_test	✅ PASS
cow_oom_test	✅ PASS

6/6 tests pass - Full CoW parity achieved.

Files Changed

• kernel/src/arch_impl/aarch64/context_switch.rs - TLB flush
• kernel/src/arch_impl/aarch64/exception.rs - CoW stats, SIGSEGV handling
• kernel/src/arch_impl/aarch64/syscall_entry.rs - TLB flush
• kernel/src/interrupts.rs - Re-export shared cow_stats module
• kernel/src/memory/cow_stats.rs - NEW: Shared CoW statistics module
• kernel/src/memory/layout.rs - Stack address validation fix
• kernel/src/memory/mod.rs - Add cow_stats module
• kernel/src/process/manager.rs - HHDM-based fork stack copy
• kernel/src/process/process.rs - cleanup_cow_frames implementation
• libs/libbreenix/src/memory.rs - sbrk page alignment fix
• userspace/tests/cow_oom_test.rs - Test design fix
• docs/planning/ARM64_TEST_CATALOG.md - NEW: Comprehensive test catalog

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