web: boot the demo before publishing it

v0.1.7 shipped a demo that could not start a session, and the site was
live and broken because nothing between `make` and `rsync` ever ran the
guest. The demo is the one artifact here whose build succeeding says
nothing about it working: it is a cross-compiled riscv64 binary in an
ext2 image, so the failures the rest of build-site would notice -- a
compiler error, a missing file -- are not where its failures live. A
guest that segfaults on startup links, packages and copies perfectly,
and the page then loads, draws a terminal and sits there dead.

web/demo/boot-check already boots that exact image in a native emulator
and fails if init does not reach a composed screen. It took twenty
seconds to find what the browser reported as one unexplained line, so
it runs in the build now, between `make site` and the copy into the
staging tree. NO_BOOT_CHECK=1 opts out; NO_DEMO=1 skips it with the
rest of the demo.

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