links: URL-scheme rules disagree across validate, links.mjs and page-checks #83

Why it matters
Three modules each decide what counts as an external link. validate.mjs accepts ftp: body links, links.mjs treats only http(s) and mailto: as external, so an ftp:// link validates but then fails the build with the misleading error link target "ftp://…" does not exist. page-checks.mjs whitelists data: hrefs while rule V16 in checks.mjs rejects data: entry URLs, so a build-only run never catches a data: URL. Also, markdown.mjs rejects a line that starts with an autolink <https://…> as raw HTML although renderInline supports autolinks mid-line.

Evidence (reproduced)

• site/validate.mjs:172 vs site/lib/links.mjs:9 vs site/lib/page-checks.mjs:83 vs site/checks.mjs:605.
• site/lib/markdown.mjs:339 /^\s*<[A-Za-z!/]/ vs :312.

Suggested approach
One EXTERNAL_SCHEME_RE exported from site/lib/shared.mjs and used by all four places; a dedicated "unsupported scheme" LinkError with the scheme in the message; change the raw-HTML guard to /^\s*<(?!https?:\/\/)[A-Za-z!/]/. Tests for ftp:, data:, javascript: and a line-start autolink.

Done when
The same URL is accepted or rejected identically by validate, build and the page checks, with one message that names the scheme.