Refactor: move Go module into v2 directory

[Alex9001]

Summary

• move the public Go v2 module into the standard v2/ major-version directory
• keep the module path, CLI install path, tags, release artifact names, and runtime behavior unchanged
• move community documents into GitHub-recognized locations and update every CI, Docker, GUI, release, and AUR build path
• add a CI assertion that prevents source files from drifting back into the repository root

Why

GitHub now presents a concise project landing page instead of placing the README below dozens of implementation files. The root contains 13 tracked entries, while the implementation remains naturally browsable under v2/.

Validation

• go -C v2 test -race ./...
• vet, staticcheck, gosec, govulncheck, and complexity guardrails
• native Qt configure, build, widget tests, and private-engine handshake
• Docker image build and entrypoint smoke test
• GoReleaser v2.17.1 config check and complete snapshot assembly (local SBOM generation skipped because Syft is CI-provided)
• extracted source archive rebuild, tests, and dependency-license collection
• rendered AUR metadata validation