fix(site): make an unusable base path impossible, not merely documented

[SUaDtL]

Turns the BASE comment into an enforced gate.

The failure it prevents

rehypeBaseLinks prefixes every root-absolute href/src with the base. A base of "/" rewrites /diagrams/x.svg into //diagrams/x.svg — a protocol-relative URL that resolves against a different host.

Nothing 404s. The build succeeds. The link audit passes, because the target still parses as a valid reference. The page just quietly fetches from somewhere else. That is why this needed to be an error rather than a comment: "/" is the obvious thing to reach for when moving a site to a domain root, and every existing check would have waved it through.

What changed

site/base.mjs validates at import time and throws. The invariant is "" (apex) or a root-absolute path with no trailing slash, so the same guard also catches "/codeArbiter/" and "codeArbiter".

Verified by mutation, not by assertion

Setting the base to "/" fails all three independently, because each imports the module:

	with RAW_BASE = "/"
npm run build	FAILED (guard fired)
npm run link-audit	FAILED (guard fired)
vitest	FAILED — module will not load

There is no path that ships it.

test/base.test.ts pins the rejected forms and, rather than describing the hazard in prose, asserts the corruption directly: prefixing under a base of "/" produces a value starting with //.

Gate

511/511 vitest (48 files), tsc clean, build green, link-audit resolves 20,057 links across 140 pages. All four run locally this time — npm test alone is vitest only, which is what let the previous two CI failures through.

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