Design: PR preview serves the homepage and /start

[serenakeyitan]

What

PR preview already mints a unique pr-<N> host (#148). Opening that host's / is still the empty branded fallback, because nothing seeds tornado-doc. Reviewing the homepage or /start means waiting for merge, or uploading to a personal Worker.

Each preview job should upload the same payload production uses:

• node bin/tdoc-landing-release → slug tornado-doc (so / is landingResponse, not a side slug)
• tdoc-start as /start
• the existing conway-life fixture for overlay chrome

Sticky Open this is the preview host root. Tutorial and overlay demo are extra links.

Not doing

• Personal BYOK Workers (including *.serenatan.workers.dev)
• Using tdoc.dev as the review host
• Publishing as tdoc-home (that leaves / on the fallback)
• Per-PR isolated landing slugs (preview KV is shared; last seed wins)

Journey

1. Open a PR on tornado-doc/tdoc.
2. The preview job comments Open this: https://pr-<N>-tdoc-preview.<account>.workers.dev/.
3. / is the landing page from that PR's checkout ("slug":"tornado-doc", isLanding).
4. /start is the tutorial. Overlay demo is still /d/conway-life/v/2.
5. Do not use the un-aliased tdoc-preview.<account>.workers.dev host — that is not this PR's overlay.