Proposal: a §9 conformance corpus (valid + invalid example bundles + a runner)

`okf/tests/test_document.py` unit-tests `OKFDocument` on inline strings, which is
great for the parser. What the repo does not yet have is a corpus of whole example
bundles that pins down the bundle-level conformance rules in SPEC §9. I would like to
contribute one, if it is welcome.

**What it is.** A small `tests/conformance/` corpus plus a parametrized
`test_conformance.py` that validates each bundle with the existing `OKFDocument` (no
new dependencies):

- `valid/` bundles that MUST be conformant: a spec-minimal one (only `type`), a full
one (recommended fields, a file-relative cross-link, `# Citations`, `log.md`,
`okf_version` on the root index), a unicode/CJK one, a broken-link one (§5.3:
consumers MUST tolerate broken links), and an extra-keys one (§4.1).
- `invalid/` bundles that MUST be flagged, each isolating one violation: missing
`type`, empty `type`, no frontmatter, unterminated frontmatter, non-mapping
frontmatter, and a `README.md` left frontmatter-less (it is not a reserved name per
§3.1, so it reads as a malformed concept — a gotcha real producers hit).

It tests the **spec's** §9 bar (parseable frontmatter mapping + non-empty `type`),
which is deliberately looser than `OKFDocument.validate()`'s producer bar (that also
requires title/description/timestamp), so spec-minimal producers are not failed.

**Why it helps.** A conformance corpus is how a young spec pins down what it actually
means: it documents §9 by example, catches parser regressions, and gives anyone
writing a producer a concrete target to test against. It slots into the existing
`okf/tests/` pytest setup and adds no dependencies.

**Two questions it surfaces, that I would align the corpus to your answers on:**

1. Does a file with *no* frontmatter at all violate §9.1 (no block) or just §9.2
(no `type`)? `OKFDocument.parse` treats a no-frontmatter file as an empty mapping
rather than raising, so today the corpus catches that case via the missing-`type`
check. If §9.1 is meant to require the *presence* of a block, the check (or
`parse`) may want to distinguish "no block" from "empty block."
2. Reserved-file shape (§6 `index.md`, §7 `log.md`) is not machine-checked yet. Worth
a follow-up, or out of scope for §9 conformance?

I have the corpus and runner drafted and passing against the current `OKFDocument`
(5 valid bundles conformant, 6 invalid flagged). Happy to open the PR and sign the
CLA if the direction is agreeable. Context: I work on Nowledge Mem, which exports OKF
bundles, so this came out of validating real output.

Thanks for publishing OKF.

Wey Gu