fix(memory): 64-bit page-size mask + ArenaAllocator move semantics

[JeanPhilippeKernel]

Root cause

The Windows crash (m_upload_pool allocation, access violation writing at a null pointer) traces to m_mem_page_size being declared unsigned long, which is 32-bit under Windows' LLP64 data model (64-bit on macOS/Linux LP64).

The page-align mask:

(offset + size + m_mem_page_size - 1) & ~(m_mem_page_size - 1)

computes ~(page_size - 1) in 32-bit unsigned long, then zero-extends that 32-bit pattern to 64-bit when ANDed against the 64-bit size_t operand. Once offset + size crosses 4 GB — exactly where this project's sub-arena budget total lands (VFS 64MB + AssetManager 1GB + Input 4MB + ECS 512MB + ImportPipeline 3.5GB + UIContext 128MB ≈ 5.19 GB) — the zero-extended mask clears bit 32, collapsing commit_size to a value smaller than what's already committed. The subsequent commit_size - m_committed_size underflows, and the garbage size passed to VirtualAlloc(MEM_COMMIT) fails, returning nullptr — which secure_memset then dereferences.

Fix

• m_mem_page_size is now size_t everywhere: the Allocator.h field, Initialize's parameter, and the local in MemoryManager.cpp. All arithmetic sites (ArenaAllocateRaw, Resize, CreateSubArena) now do pure 64-bit arithmetic.
• Both VirtualAlloc(MEM_COMMIT) call sites now commit the full [base, commit_size) range each time instead of the delta from m_committed_size — committing an already-committed page is a documented Windows no-op, so this removes the only other place a stale/incorrect m_committed_size could produce an invalid commit size.
• CreateSubArena page-aligns the sub-arena start on every platform (was Windows-only): correctness-required on Windows for clean VirtualAlloc boundaries; on macOS/Linux it costs at most one page of padding per sub-arena in exchange for a single code path instead of two.
• ArenaAllocator and PoolAllocator now delete their copy constructor/assignment — a shallow copy would double-VirtualFree/munmap the same pointer at destruction. ArenaAllocator gains move construction/assignment so it can still be relocated safely. Updated the one call site that copied MemoryManager::MainArena by value (allocator_test.cpp) to use a reference instead.

Test plan

• All 532 existing tests pass (macOS)
• Manual: engine starts on Windows past the m_upload_pool allocation without access violation