core/arena: use clear() for zeroing, pre-alloc stack to 1024, isolate prefetcher from arena

[tellabg]

Fixes

• clear() for zeroing: Replace byte-by-byte zeroing loop with clear() which compiles to optimized memclr
• Stack pre-alloc 1024: Pre-allocate EVM stack to 1024 entries (EVM max) to avoid heap escapes via append
• Prefetcher isolation: Set Allocator=nil for prefetcher goroutine to prevent data race on shared BumpAllocator (not thread-safe)
• goimports formatting: Fix lint issues in interpreter.go and arena_integration_test.go

Testing

• All arena/vm/runtime/integration tests pass
• go run ./build/ci.go lint → 0 issues
• 10,000 Hoodi blocks imported with --vmarena successfully