site: direct per-platform downloads, and bring the copy up to date

[OrellBuehler]

Direct downloads

Release assets carry their version in the file name (Kerf_0.19.1_x64-setup.exe), so
GitHub's stable releases/latest/download/<name> URL cannot address them — which is why
the site only ever linked to the releases page, where a visitor who does not use GitHub
has to work out which of eleven files is theirs.

site/assets/js/download.js resolves the real asset URLs from the API instead:

• The hero gets a split button retargeted at the visitor's platform
  (Download for Linux · v0.19.1 · AppImage · 82.8 MB), with a menu holding every
  published file grouped by OS, each with its real size.
• Get it gets three platform cards — macOS (Apple silicon / Intel), Windows
  (.exe / .msi), Linux (AppImage / .deb / .rpm) — each with the requirements
  that actually apply to it. The detected one is badged.
• Mac architecture comes from the GPU: Safari exposes no architecture hint and every
  Mac still reports MacIntel, so the WebGL renderer string decides. Unknown keeps
  Apple silicon, with Intel one click away.
• It picks the newest release carrying all three platforms rather than
  /releases/latest: while a release builds, its slowest bundle has not been uploaded
  yet, and offering that release would strand whoever is on the missing platform. This
  was verified live against v0.19.1 — the Windows rows hid themselves while that job was
  still running, then filled in.
• Progressive enhancement throughout: every link ships pointing at the releases page
  and is rewritten in place, so a blocked script or a rate-limited API is no worse than
  what the site did before.

Copy brought level with what shipped

Tool count 59 → 78. New feature cards for delivery frames + safe areas,
cut-to-the-beat, edit-by-transcript and GPU-accelerated export; rewrote playback
(streamed composited video, not a slideshow), analysis (transcription in every build),
effects (looks presets) and 360 (Insta360 pair stitching). The agent section now leads
with staged edits — the third card and the session mock show the review diff, since
agent edits no longer move the open cut. The README's feature table, tool lists, download
section and roadmap follow.

Fixes

• og:image was broken. It pointed at docs/img/screenshot-editor.png, a file that
  was never captured, so every social preview 404'd. docs/img/og.png is generated from
  docs/og-source.html; the regeneration command is in docs/img/README.md.
• Feature strings were double-escaped, rendering as Analysis & speech.
• On mobile the nav hid every link including the download chip.
• An open download menu was painted over by the band below it (every .wrap sits at
  z-index: 1, so DOM order won).

Design

A stat strip under the hero, a nav download chip, :focus-visible rings that were
missing entirely, an h1 that no longer wraps to three lines on desktop, and stat/copy
alignment.

Still open

The site has no real app screenshots — docs/img/README.md has the shot list, and the
README shows broken images for all three.