Remote control + per-PDF resume via Solid state docs

[melvincarvalho]

What

Two Solid-native features that turn the pod into a shared state layer for the PDF reader:

1. Remote control — viewer writes its current `{pdfUrl, page}` to `/public/pdf/state.jsonld` on every page change and subscribes to that doc via JSS's legacy WebSocket. Any other client that PUTs the doc (the bundled CLI, a phone, `curl`) drives the viewer in real time.
2. Per-PDF resume — viewer also writes a sibling `.state.jsonld` next to each PDF. On open, the viewer reads this and jumps to the last-read page. Owner-write, public-read inherited from `/public/.acl` — no special ACL.

The two layers are independent: remote control drives which page is active right now; resume remembers where each PDF was across sessions.

Why

Generic PDF readers can't do this — it's the pod being a real-time state bus, not just file storage. Live demo without any peer-to-peer setup: open the viewer in one window, run `pdf-remote next` in a terminal, watch the page flip.

Implementation sketch

• `src/app.js` gains `ensureStateDoc`, `pushState` (debounced), `subscribeState` (WS with reconnect/backoff), `loadResumePage`, `perPdfStateUrl`. ~90 lines.
• `cli/pdf-remote` — 30-line bash script using `curl + jq` for GET-modify-PUT. Defaults to localhost:5444; `PDF_STATE` env var to point elsewhere.
• ACL written as JSON-LD (JSS is JSON-LD-first; rejects Turtle for writes).
• README updated with the CLI snippet and a "Solid-native" features section.

Test plan

• Viewer flips pages in real time when CLI runs `next | prev | page N` from a separate terminal
• Per-PDF resume: open PDF, flip to page 12, close tab, reopen → lands on page 12
• Switching between two PDFs preserves each one's last-read page
• WS reconnect: if jspod restarts, viewer reconnects within ~10s