Add HTTP 308 Permanent Redirect status

[davesnx]

Problem

Httpev_common.reply_status cannot represent HTTP 308 Permanent Redirect.
Its built-in redirect variants stop at 301 and 302. Custom is not a safe
replacement because it requires callers to provide a full protocol-specific
status line such as HTTP/1.1 308 Permanent Redirect.

This blocks adapters that must preserve the request method while redirecting to
a canonical URL. The merged server-reason-react router now returns a
ServerEngine.PermanentRedirect outcome for trailing-slash canonicalization.

Proposed change

• Add Permanent_redirect to Httpev_common.status_code.
• Map it to integer status 308 in status_code.
• Render it as 308 Permanent Redirect in show_status_code.
• Add unit coverage for the integer and HTTP status-line forms.

Until this is available, the Ahrefs API adapter must degrade router
canonicalization redirects to 301.