Use the shared README and icon on NuGet; document manifest sharing - #13

Result

NuGet now uses the same README as GitHub and the requested MagicEF wizard icon. Publishers and users also get a complete guide to sharing and cloning MagicQuant manifests.

• Replace assets/icon.png with a byte-for-byte copy of the supplied 128x128_compressed.png.
• Package the root README.md instead of a separate shortened NuGet README, and remove that duplicate. Use canonical absolute links in the root README so they also work from NuGet; documentation checks still validate those paths against the checkout.
• Verify the nupkg's README/icon bytes and NuGet metadata in the installed-package smoke test.
• Add docs/manifests-and-cloning.md, linked from the README and existing guides: upload layout, each JSON file's purpose, --source-repo for another HF model repository, local/pinned raw JSON inputs, compatibility/imatrix limits, attribution and cloned-release evidence.

No numerical/runtime behavior or publication triggers change. Published NuGet versions are immutable; the new README/icon will ship with the next release.

Validation

• Eight Python documentation/versioning tests passed.
• Release pack completed with warnings treated as errors.
• Actual nupkg installed from an isolated local feed; asset/metadata byte comparisons, CLI startup, config generation/no-overwrite, and read-only path checks passed.
• Source/destination icon SHA-256 values match.
• git diff --check passed. GitHub CI provides cross-platform validation.