hopper 0.2.1

Hopper 0.2.1

Fixes onboarding failing with unexpected backup entry type for installations
containing sparse files, including Minecraft world region files.

• Accept GNU sparse-file archive entries and verify their complete logical contents,
  including zero-filled holes, against the original file hashes and sizes.
• Preserve strict verification: mismatched contents/sizes and unsupported special
  entry types still fail. Unsupported-entry errors now identify the path and type.
• Add regression coverage for extended sparse headers, fully sparse files, real
  onboarding restore, destination verification, and corruption rejection.

If onboarding failed at backup verification in 0.2.0, the original installation was
not moved or deleted. Update with hopper self-update, verify hopper --version
reports 0.2.1, keep the source server stopped, and rerun the original onboarding
command. Do not treat the previous unverified archive as a verified backup.

This patch does not change onboarding's conservative live-port check.

Verification: all 600 automated tests pass; Clippy and formatting checks pass.
The release includes a statically linked Linux x86-64 binary and SHA256SUMS.