Add reviewed DLL registry and pin merged upstream source

[Jacoby6000]

What changes

• Add a separate DLL registry at dll_db/index.json, starting with UnchainedPlugin 0.5.0. Each entry includes its download URL, SHA-256, size, and version.
• Add an issue template and workflow for registering DLL releases. Every DLL version goes through a PR. If a release has multiple DLLs, the bot asks the requester to add the filename and retry.
• Verify DLL downloads by default. DLL issue processing and PR checks require verification; failed issue validation blocks commits and PR creation.
• Pin the merged upstream registry implementation and remove upstream.patch. The registry code now lives upstream instead of being patched here.

Tests

All 34 local tests pass. Live dry runs also passed for UnchainedPlugin stable and RC releases, including explicit selection of normal and debug DLLs. Dry runs left the indexes unchanged.

The upstream changes are merged in Chiv2-Community/zero-infra-mod-registry#4. Launcher changes are separate. DLL review and hash checks do not sandbox native code.

[mergify]

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