Refactor PEFile and PEHeader to use ReadOnlySpan exclusively with zero-copy buffer sharing (#2317)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: brianrob <6210322+brianrob@users.noreply.github.com>
Co-authored-by: Brian Robbins <brianrob@microsoft.com>

Author: Copilot