fix(memory): bound editor and upload arena ownership

[JeanPhilippeKernel]

Summary

• reserve a bounded EditorContext owner before editor initialization and move editor-owned CPU allocations into it
• materialize the VulkanDevice CPU budget and move renderer decode slabs into the bounded import/upload owner
• validate built-in profile capacities and align allocator and memory-budget documentation with actual ownership

Validation

• cmake --build --preset Darwin_arm64_Debug --target ZEngineTests Obelisk
• ZEngineTests --gtest_filter=AllocatorTest.:MemoryBudgetConfigTest.:TLSFSlabTest.:WorkerThreadCountTest. (49 passed)
• launched the Debug editor with SampleProject/project.json for 30 seconds; it remained alive past the former allocation failure
• git diff --check and repository pre-push clang-format checks

Scope

This resolves the missing-owner startup path in #834. The separate POSIX reserve/commit redesign, complete MainArena ownership audit, and measured upload-concurrency policy are tracked in #835.

Fixes #834