chore(brand): drop the unreferenced legacy light/dark logo fork

[chrisjleal]

Why

images/proofable-mark-black.svg is a leftover of the retired light/dark logo split. docs.json now points both themes at the single Arctic mark:

"logo": { "light": "/images/proofable-mark.svg", "dark": "/images/proofable-mark.svg" },
"favicon": "/images/proofable-mark.svg"

The file has zero references anywhere in the repo (checked across docs.json, all .mdx, and assets), yet it was still tracked. neus's brand-asset contract guards this directly:

expect(fs.existsSync(path.join(docsImages, 'proofable-mark-black.svg'))).toBe(false);

That assertion was failing on the current tree — a real cross-repo drift, not a flaky check. Removing the stale fork satisfies the contract and restores one mark with one owner.

Evidence

• git ls-files -- images/ no longer lists it; the other four marks are untouched.
• rg "proofable-mark-black" returns no matches repo-wide.
• docs working tree is otherwise clean.

Docs assets only — no page, config, or copy changes.