Opening a file in an app-owned (GitHub) vault could show a black screen: the
reader loads a rendered `<doc>.html` sibling via WKWebView.loadFileURL (so
relative images resolve), but the sibling is written just after first paint
(.task) — so the first load hit a missing file, and any file:// read-access
failure left the transparent web view over a dark background (= black).

Make the app-owned render path self-healing, without losing relative-image
fidelity:
- ReaderView passes the in-memory `html` to the app-owned ReaderWebView as a
  baseline (the key still keys off the file URL, so reload behavior is unchanged).
- ReaderWebView loads the on-disk file when it exists; otherwise it renders the
  in-memory HTML immediately (never blank), then upgrades to the file once the
  sibling is written (token bump).
- A new didFailProvisionalNavigation degrades to the in-memory HTML if a file
  load fails (read-access/not-found), ignoring deliberate link-interception /
  external-open cancels.

The HTML baseline is the same path folder vaults already render successfully, so
the reader can no longer black-screen; loadFileURL still provides relative images
when it succeeds.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>