docs: announce stable X8 release and add Discord contact

[vntrevx]

v1.16.0 is now the latest stable GitHub release, so users can upgrade to Native X8 support with nfi-bte update. Replace RC-only installation guidance in the README and English, Korean, Greek, and Turkish usage guides with the stable update path, and use X8 in the walkthroughs while documenting the X7 option. Update the release notes with completed qualification and promotion results.

Add a centered Discord-colored badge and an English invitation at the very top of the README: readers with issues, ideas, or contact requests can send a Discord DM to vntrevx.

Validation:

• Stable promotion workflow 34228035469 passed, including exact fixture, byte-identical asset promotion, and Linux/macOS latest installer audits.
• All 13 stable asset names, sizes, and SHA-256 digests match v1.16.0-rc.1.
• An isolated installed v1.15.0 successfully upgraded to v1.16.0 with nfi-bte update; the subsequent check reported current.
• Documentation text validation, local link checks, 47 changed shell example syntax checks, and git diff --check passed.

Documentation only. Required CI should select the docs-only lane. Preserve existing release binaries, RC evidence, and certification boundaries; no long backtests are needed.

[chatgpt-codex-connector]

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