chore: refine xray-only wording

[besnow]

Motivation

• Remove misleading “multi core” wording and make the repository's xray-only positioning explicit while avoiding any behavior or dependency changes.
• Ensure README clarifies that this is an xray-only fork (runtime uses Xray core only) without implying removal of Xray-core internal capabilities or restoring external cores.

Description

• Update cmd/version.go to change the intro string to A V2Board backend based on Xray core (wording-only change).
• Rewrite README.md sections to explicitly state the fork's xray-only positioning, preserve internal Xray-core capabilities, avoid restoring external sing-box/hysteria/hysteria2 cores, and limit production claims to canary/gray validation.
• Changes are strictly textual and scoped to cmd/version.go and README.md; no functional code, module, ACME/CertMode, Actions, or Dockerfile edits were made.

Testing

• Ran gofmt -w cmd/version.go successfully.
• Ran go build ./... and the build completed successfully.

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Codex Task