docs: badge row, and a test that the links in it are real

[darklordVirtual]

Four badges at the top of the README — CI, the research repository, the paper, and the licence — so a developer sees what this is built on before any prose. "Powered by REMORA" became a sentence that states the actual relationship: consumed as a hash-pinned release, not a dependency on its master.

The test is the interesting half

A dead link on a public front page reads as neglect of everything behind it, and the specific hazard here has already bitten once: docs/ARCHITECTURE.md and docs/architecture.md are the same file on Windows and two different files on the Linux host GitHub serves from. Three links resolved perfectly for the person who wrote them and would have 404'd for every reader.

test_readme_links.py checks every relative link in every document against the real directory listing, component by component.

The first version of it did not work — and failed in exactly the way it was written to prevent. It called Path.resolve(), which on Windows canonicalises a path to the real on-disk spelling, so docs/ARCHITECTURE.md came back as docs/architecture.md and the check validated the lookup rather than what was written.

Verified by deliberately breaking a link:

first version   24 passed          ← did not catch it
rewrite          1 failed, 23 passed
                 README.md->docs/ARCHITECTURE.md
restored        24 passed

It also asserts that a CI badge names a workflow that exists — a badge for a missing workflow renders as "no status" and reads as a broken build — and that no document is unreachable from the README.

Filesystem only: no network, no Docker, part of the fast gate. 95 contract tests pass.

Note on ordering

Branched from main so the badges can land without waiting on #6. #6 also touches the README; I will merge main into it once this lands so it stays mergeable.