Add landing page + Caddy download host (site/)

Self-contained, no-JS landing page (tinygrad-minimal) that doubles as the
artifact download host, served by Caddy on an internal :80 box.

- site/index.html: static page, "Same tool, only a little different", feature
  highlights (remote->remote, symmetric panels, ssh_config awareness, edit-in-
  place, self-copy guard, ...), fixed stable download links, FAQ (unsigned notes).
- site/Caddyfile: one flat /dl/ folder with browse + range requests and per-type
  Cache-Control (patches immutable, update.json no-cache, installers short).
- site/deploy.sh: drop a build's artifacts/stable-* into /dl/ (renames the win
  portable zip to a stable name).
- Flat folder matches electrobun's artifacts/ layout, so it's also the future
  updater baseUrl; per-build folders/symlink dropped in favor of it.
- docs/build-and-release.md: document the download host + deploy flow.

Verified: caddy validate + live smoke test (links, cache headers, 206 range,
404 fallback, deploy.sh copy/rename) against a temp root.

Co-Authored-By: Claude <noreply@anthropic.com>

site/assets/README.md

@@ -0,0 +1,11 @@
+# Image assets
+
+Drop images here as they're ready; the page references them by relative path.
+
+Planned slots (commented into `../index.html`):
+
+- `logo.svg` — hero logo above the wordmark (`.logo`, 128×128). Source lives at repo
+  `assets/logo.svg`.
+- `screenshot.png` — product shot under the intro paragraphs (`.shot`).
+
+To wire one up, uncomment the matching `<img>` in `index.html`.