TOC: Ctrl/Alt + Down/Up step the selection through the tree in document
order (App.tsx flattenToc + a global keydown), and clicking a TOC entry
now hands focus to the RHS pane (selectFromToc, <main tabIndex={-1}>) so
the keyboard drives the viewer immediately. Ctrl, not PgDn/PgUp, because
Ctrl+PgDn/PgUp is a browser tab shortcut the page can't override; Alt
aliases Ctrl for macOS Mission Control.

PDF/DjVu: focus the .filepdf/.filedjvu scroller on mount (it is a nested
scroll container the focused pane can't reach) so arrows / space /
PageUp-Down / Home-End scroll the document natively. Skipped when the
viewer is embedded as a chapter chunk.

Explorer: roving-focus icon grid — plain arrows walk the members (Up/Down
pick the nearest item in the adjacent row from live geometry, correct for
the wrapping flex grid), Enter opens the selected one; Ctrl/Alt+arrows are
left for the TOC nav above.

README: directory-view + chapter screenshots.

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