Docs: point install instructions to 0.1.1-beta.15.4

[HANCORE-linux]

Summary

• Point the install, update and recovery checkout instructions to the published v0.1.1-beta.15.4 prerelease.
• Extend the documented predecessor range through v0.1.1-beta.15.3; update the package identity and current host references. Keep all 24 plugins and AUR publication deferred.
• Documentation only: six Markdown files, including the mechanical host-reference update in docs/architecture/quattro-compatibility.md and the next-release reference in docs/release-readiness.md.
• Leave historical acceptance records unchanged. No code, test, pin, CHANGELOG or release-note changes; no new release or tag.

Validation

• One local python3 tests/test_package_release.py run: 27 tests passed.
• Reviewed complete diff; verified exact commit metadata, unchanged source modes and clean worktree.

The published beta.15.4 tag and assets remain unchanged. This documentation commit is based on main merge 67d87c5d7f61fca4b05e79a089238805e0e0737f.