Mindrealm v1 revision r13: native SHA256/SHA512 checksum module resolution

- Previous revision: r12. Next revision: r13.
- Normal built-in PowerShell Get-FileHash is the ONLY hashing implementation
  used for new package sidecars and content verification.
- Shared tools/Require-NativeFileHash.ps1 forces Microsoft.PowerShell.Utility
  to load. It can import using the $PSHOME manifest when PSModulePath is damaged.
- A missing native cmdlet causes a clear, early error; it is never faked,
  replaced, or silently skipped.
- Generated End User distributions still receive both ZIP.sha256 and ZIP.sha512.
- Original r12 release checksums are immutable; new r13 artifacts receive new digests.
- Shared compiler and bootstrap engines remain byte-identical across projects.
- Run tests/Native-Get-FileHash.Tests.ps1 under Windows PowerShell 5.1 or pwsh 7.

NOTE: The historical PHLEGM r19 portable-hash compatibility file is retained
solely as a legacy/test artifact; active PHLEGM entrypoints no longer dot-source it.
