fix(difftastic): support versioned release assets

[jsburckhardt]

Summary

Difftastic 0.71.0 added the release version to archive filenames. The installer still requested the legacy filename, downloaded an HTTP 404 response, and failed during tar extraction.

• Resolve download URLs from GitHub release metadata using exact matches for both legacy and versioned filenames.
• Fail explicitly on HTTP errors and missing or ambiguous matching assets.
• Include tar in installation dependencies and bump the feature version to 1.0.1.
• Keep the existing pinned 0.63.0 compatibility scenario and add a pinned 0.71.0 regression scenario.

Validation

• Latest release feature test on ubuntu:latest: passed (rerun in isolation after concurrent test cleanup interference).
• Latest release feature test on debian:latest: passed.
• Latest release feature test on mcr.microsoft.com/devcontainers/base:ubuntu: passed.
• Global difftastic-specific-version scenario (0.63.0): passed.
• Global difftastic-versioned-asset scenario (0.71.0): passed.
• Bash syntax, JSON parsing, and git diff --check: passed.

The pre-existing local devcontainer lockfile change is not included. CI continue-on-error settings are intentionally unchanged.