Artist ordering demo: configure, check, offer, fan-funded campaign

New static page docs/artist-order.html (test site /demos/artist-order/)
walking the artist platform flow end to end:

- record configurator (format, speed, discs, weight, colour, packaging,
  extras, mastering, test pressings) with a live mock-up
- track list or dropped masters read in the browser (WAV/FLAC/AIFF
  headers, sample peak and clipping), side length against the format,
  jacket and label preflight (size, dpi, CMYK), rights and gate 1
- itemised offer (per title / per copy, pass-through jacket), price per
  copy against quantity, lead time from the production timeline
- direct order or fan-funded campaign: goal = max(minimum run,
  break-even), what the artist keeps, the money rules, a simulated
  campaign (sells out / just makes it / falls short) with refund or
  go-ahead-at-minimum, and the production tracker with the three gates
  and the fan update feed

Price list demo-0.1 splits the doc's 38,000 THB per title and 132 THB per
copy into lines (estimates, each listed with its basis); defaults
reproduce the artist platform example (break-even 41, goal 100, ~548 THB
kept per copy at 100).

artist-platform.md: demo paragraph, side length per format table with
sources, VAT note on the campaign example. README bullet and layout line,
site DEMOS line and landing-page entry.

site/build.sh

@@ -21,6 +21,7 @@ DEMOS=(
 	"docs/plant-operation.html demos/plant-operation/index.html"
 	"docs/fleet-calculator.html demos/fleet-calculator/index.html"
 	"docs/steam-cooling.html demos/steam-cooling/index.html"
+	"docs/artist-order.html demos/artist-order/index.html"
 )
 for entry in "${DEMOS[@]}"; do
 	read -r src dst <<<"$entry"