docs: align published Server 521 release entry points

[chen21019]

Scope

Docs only: add the published Server 1.6.521 release/index links, immutable digest and compatibility tuple (Web 1.6.181 / Engine 0.183.337), and update the performance example to the numeric 1.6.521 install tag. Historical sections remain intact; no full-matrix or C-03/C-05 live acceptance claim.

Verification and runtime boundary

• Exactly five Markdown files changed; relative targets and official release digest checked.
• server/ and scripts/ Git tree IDs are byte-identical to current main. No runtime or source-gate change.
• The follow-up commit normalizes only worktree CRLF to the pre-verified Git index; the final tree matches that index exactly.
• No product test, build, publication, deployment or merge performed for this documentation PR.
• Server522 producer fde3cef761e45392fa364a4afda8a9c6b12f161b and its existing 22-layer candidate image remain unchanged. This PR does not merge build: prepare Server 1.6.522 with Engine 338 and Web 182 #274 or claim Server522 is published.