Move repo facts outside the project payload

Repos need not carry a `xyz.radicle.project` payload; the default
branch can come from `xyz.radicle.crefs` alone. Such repos got no
counts and no default branch, and failed schema validation in
explorer, which broke the parse of a whole listing.

httpd:
- Add top-level `cobs` with issue, patch and bucketed release counts
- Add top-level `defaultBranch`, qualified like the crefs payload
- Omit a count the node cannot determine instead of reporting zero
- Drop `head`: the tip is `refs[defaultBranch]`
- Keep counts in the project payload for older clients until a
  major release
- Keep the project payload when a COB read fails
- Keep `meta.releases` a single count: delegate + other
- Log and skip canonical ref rules and refs that fail to read,
  so one bad ref doesn't drop the default branch tip

explorer:
- Model `cobs` and `defaultBranch`, adapting older nodes to one shape
- Resolve the default branch from the crefs HEAD on older nodes
- Collect the shims for older httpd in http-client/lib/compat.ts,
  each naming the httpd release that the client can drop it with
- Add `defaultBranchTip`, replacing `head`, with a fallback to the
  legacy `meta.head`
- Make the project payload and its legacy meta optional
- Fall back to the RID for the repo name, and show a placeholder
  for a missing name in repo cards and the header
- Hide a counter instead of reading a missing count as zero
- Fall back to a full-page check for "More" without counts
- Add a `repo.sourceUnavailable` route for repos without a
  default branch or tip
- Browse explicit revisions and offer the branch selector without
  a default branch
- List these repos in repo grids and on the landing page
- Say the node is older or built without releases
- Scope release pages in the client, so views never call the shims
- Read OG worker repos through the same compat shims
- Share the repo name and default branch helpers with the OG worker

CI:
- Add check:open-graph, a wrangler deploy dry run
- Run it in GitHub CI, Buildkite and check:ci
- Install explorer's dependencies first: the worker
  resolves compat.ts imports, such as zod, from there

Tests:
- Test that a build without releases omits the count
- Test that one failing rule keeps the others
- Test the repo shape across httpd versions and the release shims
- Test `hasMorePages` and `repoName`
- Test repos without a project payload end to end

.github/workflows/explorer-build.yml

@@ -43,6 +43,12 @@ jobs:
         run: npm run build
       - name: Run Playwright build smoke test
         run: npm run test:e2e -- --project chromium --config playwright.buildSmoke.config.ts
+      # The worker resolves compat.ts imports, such as zod, from explorer's
+      # node_modules, so it needs both installs.
+      - name: Install Open Graph worker dependencies
+        run: npm ci --prefix workers/open-graph
+      - name: Bundle Open Graph worker
+        run: npm run check:open-graph
       - name: Upload artifacts
         uses: actions/upload-artifact@v7
         if: always()